Shit like Tera Steel Volc, Tera Fire Kingambit fundamentally flip common counterplay on their heads and use those turns to snowball to a point where counterplay no longer exists.
Except Volc and Kingambit have kits that have actual resistances and choose their tera to flip specific matchups against those kits. Iron Hands is always going to resist Kowtow Cleave and Iron Head. Whatever Tera Type Kingambit runs, if I'm running Iron Hands on my team, I know I'll be able to switch in and make progress. Volc running Giga Draing is always going to struggle with limited coverage, and its Physical defense will always be exploitable, even on bulky sets. I can always switch in Dragonite, for instance, and boost up along side it before KOing with ESpeed (or coverage, if it reveals its tera type).
You can't say the same thing about Garg. It doesn't care what your type is, unless you're Water or Steel, in which case it just hates you a bit extra. All Garg does is wear you down unless you're running Cloak or Sub. And even if you are running cloak or sub, you don't know what else Garg is running besides Salt Cure and Recover. Because it doesn't care about type matchups offensively, it also gets to not care about them defensively. Garg isn't trying to capitalize for itself, it's filling a hole on the team. And Purifying Salt means it doesn't even need to worry about things like the added burn/poison/paralysis immunity that some types grant. A builder with Garganacle literally looks at their team and says "what am I weak to that I don't want to be/what do I want to lure?" and then changes the tera type on Garg to fit, and if the other player isn't running Cloak Haze Toxapex (or PhysDef Skeledirge or maybe cloak Dondozo), Garg is probably gonna do its job.
Lets use your Volcarona Example! Are you afraid of Tera Grass/Steel Volcarona? Well, now, for the low price of running Boots instead of Leftovers, you don't need to worry about it ever again.
+3 0 SpA Tera Steel Volcarona Fiery Dance vs. 252 HP / 252+ SpD Tera Fire Garganacl: 108-127 (26.7 - 31.4%) -- guaranteed 4HKO
+1 0 Atk Tera Fire Garganacl Tera Blast vs. 248 HP / 108 Def Tera Steel Volcarona: 318-374 (85.2 - 100.2%) -- 6.3% chance to OHKO
Obviously, Volc could be running Ground or Water with Tera Blast and then you're out of luck, but Volc is also arguably a top 5 tera abuser in the tier (and one of the few that's actually reasonably likely to run Tera Blast).
I get that setup sweepers getting a free turn probably costs more games and is arguably less "competitive" in that way. I've done my share of clicking Tera Poison on SD Chien-Pao and winning games I should have lost. But even if you "know exactly what Garg is going to do" and can "play around it", you still have to kill it at some point to win the match, and it's such a blank slate (again, because it *wants* to get rid of its typing and it doesn't particularly care about maximizing its existing coverage) that you may not know that you're best counter to it is gone until it already is or that that unassuming Garg on preview is going to wall your best setup sweeper to hell and back.