As a Lance main: The Lance isn't at all built to 'tank' things because MHW, indeed, does not have tanking. The base game doesn't have an aggro system, and 'tanking' doesn't just mean taking damage, it means specifically maintaining a mob's attention to create space and time for your teammates.
The Lance is, like most weapons in the game, intended to deal damage to the enemy. It does that by sticking to the enemy and not backing off or giving ground even when being attacked, which is enabled by its guard, counterattacking, its ability to quickly reposition, and its ability to follow the monster no matter where it goes with the charge. The role that it excels at, the niche it feels, isn't 'defensive' per se—it's a form of offense that is founded on constant, unabating aggression, which is itself the product of single defensive actions that themselves create opportunities for more offense. This distinguishes it from other melee weapons that have to sacrifice space or time in response to the monster's own offense.
"Poke things, guard when an attack is coming, then keep poking things" isn't tanking, it's an attack pattern. It's one that does allow the Lance to more easily fill the 'tank' role in the Behemoth fight specifically, but in any other context the Lance doesn't "tank." It attacks differently.