>>49590942>>49591010It's used so much because it's amazing team glue. It compresses a lot of important qualities a team might need (ground imminuty, electric immunity, soft-check to most physical attackers, reliable pivot, rocks setter or remover) and it does that stuff fairly reliably in a myriad of different matchups.
Since it has such a unique typing there aren't really a lot of other Pokemon aside from Gliscor that can fulfil this kind of role. Gliscor isn't even in gen 8 but if it was it would almost certainly be used a lot too considering the most widespread Landorus-T set right now is a bulky Leftovers set.
Despite the high usage Landorus-T is not problematic to deal with at all. It doesn't have access to a secondary STAB, it has no reliable recovery, it's not really that fast unless it's scarfed in which case it's inherently limited in its move selection, it has a blatant double weakness to a good attacking type and most importantly it's not a Pokemon that really restricts your teambuilding or decision-making. Landorus-T being in the tier does not force everyone to only bring very specific countermeasures or risk auto-losing to it. The vast majority of teams will have a good ground immunity, some strong ice or water attack, naturally faster Pokemon without specifically having to make ridiculous sacrifices just to answer this one specific threat. Landorus-T doesn't auto-win because you didn't bring X or Y or because you lost one "coinflip" decision early on in the battle.