I think you've really got to look at how Garchomp performs in terms of the metagame - its Ground/Dragon typing allow it to take on Sun and Sand really well; Salamence and Dragonite have trouble taking on Sun between Sun boosted attacks and their weakness to Stealth Rock, and a lot of popular sand abusers (like Landorus in particular) carry Stone Edge (and outspeed in the case of Landorus-I) to hit them with. Of course, the counter to this is that Garchomp is hit by Earthquake and has no recovery, but this brings me to an important point, which is that maybe Garchomp cannot be compared to other Dragons/Landorus, since it has that unique Ground/Dragon typing, it has that 102 base speed, but it also has comparatively middling stats and no recovery. You just can't say that Garchomp is a Salamence or Dragonite rival.
I'll give you the lack of SR weakness as that can be kind of big. However, I kind of do view Garchomp's main rivals as physical Dragons (Salamence, Dragonite, and Haxorus) since all 4 can run band, 3 can run Scarf, and all 4 can effectively use boosting moves. They generally compete for similar roles. Yes, Garchomp has that great Ground/Dragon typing that gives it great STABs. I'm not taking that away from it (I'd question my own sanity if I did tbh) however that is also a double-edged sword. It's now hit neutrally by Water moves, meaning it has an effective Water weakness in Rain. Fighting is one of the more common offensive types in the tier and Garchomp is hit neutrally by them. Haxorus is hit neutrally as well, but its pure Dragon typing still lets it resist Water. Chomp also lacks the specialized stats and abilities of each as all 3 have a better ability
on average than Garchomp. Dragonite has Multiscale. Yes, SR screws it up, but you have Roost to reactivate it should you choose to run Roost. With Multiscale, Dnite doesn't need a Yache Berry to survive Ice moves like Garchomp does. Dnite has ESpeed to make up for its lower Speed and a slightly higher base Attack stat than Garchomp. Their defenses are very comparable. Dnite has 91 / 95 / 100 defenses while Chomp has 108 / 95 / 85 defenses. With Multiscale, Dragonite makes better use of that bulk than Chomp can on average.
Then there's Salamence. Salamence is basically the best Dragon type Scarfer thanks to its ability, Moxie. Garchomp doesn't have this awesome ability and which side of the spectrum does Salamence hit on with its Scarf set? That's right, the physical end, the same end as Garchomp. Salamence has a much higher Special Attack that allows it to do much, much more with Fire Blast than Chomp can hope to do. Ergo, Salamence has the more balanced stats and better ability for a Scarfer. Sure, it can't outspeed Lando-I like Chomp can, but you should have an answer to faster Scarfers when using Salamence anyway.
Now to Haxorus. It's arguably better with Choice sets than Chomp is since it has an ability that actually makes a difference. It has Mold Breaker. What does this mean? That same Rotom-W, or Gengar, or Bronzong that is forcing Garchomp to use Outrage or switch out instead of using EQ can't do the same thing to Haxorus. With Mold Breaker, Haxorus bypasses these Levitate users with EQ, meaning he can't get revenge killed by Ice Shard users or priority users in general as easily as Chomp can. Sure, Chomp gets STAB on EQ. That's great. You're still forced into Outrage against Levitate users and Skarmory still walls you to hell and back. Speaking of Skarm, Dnite, Mence, and Haxorus all have ways to bypass it. Dnite and Mence have the Special Attack stat to run Fire Blast while Haxorus has the raw power with rain boosted Aqua Tail to get past. (Band set only for the latter)
Finally, there's the boosting issue. Mence, Dnite, and Haxorus all get Dragon Dance. This means that they are boosting
physical attackers. Chomp gets Swords Dance. This means that he's a boosting
physical attacker. Unlikes the other 3, Chomp can't boost his Speed without a Scarf, meaning its sweep will get cut short much more easily than the other 3. Even your argument with Landorus-I revenging Dnite and Mence with Stone Edge isn't fully true since Dragonite can easily use ESpeed to bypass the faster threat and take it out should it be weakened enough. Like with ScarfMence, you should have ways to eliminate faster threats if you want to sweep with DD Dragonite.
Salamence and Dragonite are perfectly viable Pokemon to compare Garchomp to. All 3 are primarily
physical attackers and generally want to do
similar things, thus meaning that they compete for the same team slots. Of course Chomp is a good Poke. Yes, you need to prepare for it. Yes, it's powerful. But if you prepare efficiently for other physical Dragons, you'll be ready for Garchomp.
Edit @ above: Sash Mamo... LO Mamo is generally more common in my experience. Jolly LO Mamo does 100.55% - 118.43% to the same Scarf Garchomp. So, more bulk or not, Chomp still gets OHKOed by the most common Mamo set. The rest is valid, but don't forget Lando-I gets a boost to Stone Edge in Sand. EdgeQuake is a fine combo to have available to you as a Scarfer.