I won't argue that Garchomp probably won't get set up here itself, as the Pursuit user likely wants to switch out anyway. They do, however, have the option of staying in to do some respectable damage to Garchomp, forcing Garchomp to guess next turn whether to risk using their super-effective move on a resistant/immune switch-in. That's no option against Lucario, who takes an absolute pittance from Pursuit.
Adamant CBTar's Pursuit vs 4/0 Garchomp: 604 Atk vs 226 Def & 358 HP (40 Base Power): 115 - 136 (32.12% - 37.99%)
A 3hko. Garchomp gets a Swords Dance and then a free 2hko of something on your team. So yeah, they do have the option of staying in, but it will be the exact same result for them.
Also, the above poster is correct. You can't compare Garchomp and Lucario, they are two different kinds of sweepers. Since Lucario has easily exploitable weaknesses and piss-poor defenses, you basically *have to* come in on something weak that it 4x resists like Pursuit in order to set up. Lucario has to be really picky when it comes in, since if your opponent has a Salamence or Gyarados waiting in the wings, you just spoiled your big surprise. Also, Lucario is forced to lower its already abysmal defenses if it wants to sweep, and with Life Orb chipping away at its HP, this is obviously tough to compare to Garchomp, who tanks hits even with no defensive investment to get its sweep in.
Lucario and Garchomp are used in completely different manners, you can't really compare them because of how much differently they are played.
The mere fact Garchomp so frequently relies on resistance berries is also a handicap in itself. If it's using Yache Berry (or Haban), which Garchomp so often does, it's not using Leftovers to mitigate residual damage or a damage-increasing item (LO and such) to boost its power.
Garchomp doesn't rely on resistance berries, it takes advantage of them. Garchomp has the pure offensive stats to not need to use a Life Orb, so defensive items actually help Garchomp more than something like a Life Orb would by giving it the stability to be a long-term threat.
Considering that Garchomp 2hkos literally every pokemon in the game with or without an attack boosting item, I would not call using a berry to eliminate one of its few weaknesses a "handicap". Garchomp, unlike Lucario since you seem to love that comparison, doesn't need a Life Orb to get critical OHKOs and 2HKOs. Unlike Salamence, Garchomp doesn't need a Life Orb to boost its damage because it has Swords Dance. Garchomp doesn't lose anything by using a resistance berry, but it gains a lot.
Using a resistance berry to bolster its defenses (which are already better than Swampert, mind you) is far from a "handicap".
Garchomp doesn't need a Life Orb, it is complete overkill and it just sacrifices its amazing defenses with no real gain.
Speaking of defensive items, if someone was really ballsy they would use Leftovers SD Chomp over a resistance berry and turn CBScizor's Bullet Punch into a 3HKO with 0 defensive investment. That's pretty damn impressive.
Feel free to name some of them. Actually I'll name them for you: ScarfTran... Scarf Flygon? That's about it as far as common checks go, not including rarities such as 308 Speed Gliscor, the odd Mach Punch or Vacuum Wave user, or mid-range Scarf users like Metagross.
I don't know why you keep talking about Lucario, but I'll play along. Lucario will always have a 100% counter in OU. Every moveset is walled by something really common.
First of all, Gengar, anything with a Choice Scarf, Salamence, Gyarados, Skymin (not ohkod by +2 extremespeed), Latias (also not ohkod), and Rotom-A (with Speed) are all amazing checks and counters to any Lucario. If it lacks Crunch, then Rotom-A, Celebi, Gyarados and Cresselia wall it. If it lacks Ice Punch, Zapdos, Gliscor, Gyarados all wall it. But this is all irrelevant to the issue at hand anyways.
Sand Veil is considerable but I will repeat what I have said previously. The first thing to note is that Sand Stream is only active in about 50% of battles. It's not even being used half the time!
Yeah, but it IS being used half the time...that's the whole problem.
Statistically speaking, that's similar to Garchomp only having 10% evasion under Sand Veil. That doesn't sound nearly as bad as 20%, does it?
Actually it still sounds pretty bad.
Secondly, I would note that having to build around Sand Stream at all is considerable concession for a player to make. That is to say, a person "pays" for their ability to abuse Sand Veil with decreased team-building options at the onset. (See: What Nubchos said above.)
I don't see how using Tyranitar, who has been top 10 OU for literally all of DP, is a handicap at all. TTar is a great lure for Garchomp's checks. I personally used a fast Expert Belt TTar to trick people into switching their Steel-types into Crunches, only to be met with a Fire Blast for massive damage. This is especially hilarious since Scizor always U-turns out instead of locking itself into BP. TTar is supporting Garchomp by luring its counters out (good luck switching into Outrage if nothing you have left resists it). They work very well together.
Since you keep talking about how "omg Latis Starmie Azelf Jolteon are faster than Garchomp so they can beat it", why is using Tyranitar, as complete a counter as you can get to them, a handicap? Your definition is pretty messed up if using TTar is a handicap.
It's also worth mentioning that Sand Veil doesn't work on all of Garchomp's potential checks, the most notable being Toxic Spikes. You can't dodge those, sorry mang.
Toxic Spikes do slow Garchomp's sweep down, but a poisoned Garchomp is still a Garchomp, a top 10 uber. Maybe if there were Burn Spikes....
Actually, Haban Berry is junk and people should stop using it because Garchomp usually gets killed through it anyway. Yache Berry is alright, it gives Garchomp some room for residual damage in the face of defensive Ice users and it stops Ice Shard from flat-out beating it.
Although you are right about Latios still ohkoing through Haban Berry, you ignore the fact that Haban removes Garchomp's most threatening revenge killer: Garchomp. Chomp's unboosted Outrage can't ohko other Garchomp through Haban Berry. I have won quite a few matches because of this.