This is mostly just a matter of what they can do. Those two comparisons you made is mostly about utility Landorus and tank or spdef Garchomp. The reason for this is Lando has more tools to play with than Garchomp does. Uturn, kncok off, defog, among others. Aside from utility, Lando provides the crucial electric and ground immunity all in one slot and its typing can easily be covered by all the bulky waters running around. Lando's access to nearly all of the utility moves one can dream of is what makes it the bread and butter of ou. Garchomp only has access to rocks and toxic I guess which Lando also has. Most crucially is that Landorus is not weak to fairies like Garchomp is. Landorus' two immunities, utility moves and the best ability in the game is what makes it way better than Garchomp is
Of course, this only applies to their utility sets. When it comes to offensive sets, Garchomp is leagues beyond Landorus. It's arguably the most dangerous Pokemon in ou because it has two unique advantages which I wrote in detail here. I would say that even in smogon tournaments, Garchomp is fairly underexplored since lefties seems to be the standard and no one really abuses the fact that it bloody resists stealth rock. I'm just gonna tldr the post I made there but Garchomp is the only common offensive Pokemon that has more options than everything else thanks to its resistance to rocks and non spammable stabs
Also, Landorus doesn't have access to roost. If it did, this thing would be a nightmare. Imo, utility Garchomp = just use Landorus while offensive Garchomp > offensive Landorus. They both have their own advantages but since ou has a lot of offensive mons and because of Garchomp's limited utility, Landorus is preferred over it. Watching Urshifu kill itself may be very amusing but Urshifu isn't on every team which only further limits tank Chomp's utility. In all honesty, I don't even get the point why this set still exists
And for the love of all that is unholy. Don't use lefties of Garchomp. It's such a huge waste of Garchomp's potential since to an extent, it can mix and match its own counterplay that isn't Mandibuzz or Togekiss
