>>26756300A Pokémon not having counters does not make it broken. Kyurem-B does not have any counters, but it does not force you to carry a specific mon to deal with it.
Even in Landorus-T's case, that simply isn't true. Rotom-W and Skarm are full stops to any Lando variant, as well as Hippo with Whirlwind, Gliscor (barring SD sets if Gliscor doesn't run Toxic). It's checked by most special attackers like Keldeo or the Lati Twins, Breloom (Lando cannot really touch Breloom, scarf U-Turn does 30%ish so Breloom can just set up or Spore something), Bisharp, Azumarill. Its low speed means it is outsped by most scarfed OU mons as well.
Lando-T has such high usage because OU is inundated with so many powerful physical attackers. Lando-T keeps them in check, the way Jirachi kept many special attackers like the Lati twins in check in gen V. Lando-T is easily worn down by chip damage from switching in to buffer attacks and talking twelve percent from Rocks every time it switches in. This is supplemented by its lack of recovery apart from Leftovers and no priority. It's also very slow (compared to the mons it comes in on, such as Mega Lopunny). Oh, it's running Scarf? Then it's not going to be able to switch in as much and even more susceptible to chip damage (Talonflame's Brave Bird does about 30% to Scarf Landorus-T, so it really cannot come in more than once. It relies on its teammates getting up Rocks and playing aggressively to keep Talonflame out, assuming Lando-T is the only bird check on the team). Scarf variant is forced to U-Turn or switch out versus mons it cannot OHKO, such as Manaphy, so unless Lando-T is threatening the mon right in front of it it's in its best interest to get out of there.
So Landorus-T is overused because it's a buffer and a pivot for all the powerful physical attackers in OU that would tear the tier a new asshole otherwise. It's like why Rotom-W had the highest usage in early XY; it was the best bird counter in the game at the time.
