Because this game follows the same structure as earlier games and uses the same language. "Tank" originated outside of warframe and is part of the trinity in MMORPGs which includes MMOlootershooters (we also use healer/dps in this game for warframes). I think the earliest use was in 1992 according to wiki. The concept, EHP, taunt, aggro all existed in other earlier games. You use the phrase "tank" - you are on that hook.
But not in game. You are comparing things in a way that has no relationship to the way they are actually played. Hildrynn is the most glaring example quoted, but she is not most obvious.....
Nyx using absorb & it's aug + ack/brunt melee & electronmagnetic shielding aug + deth cube & energy generator aug and synth deconstruct + arcane energise + enough stretch to cover deth cube so it is immortal + rubico & sharpshooter mod (regen energy on headshot kills) + guardian derison on melee. You have a warframe that can hold aggro, redirect damage to itself (tank abilities), CC, regen energy from multiple sources and is immune to all damage. Nulifiers would be a pain, so use a secondary embolist with eroding blight for 25% energy regen on syndicate proc. Rank on your list? 44 - so a bad tank.
Why, because your list says Nyx sucks as a tank because none of that can be used.... but I use it in game and in some situations Nyx outperforms everything else as a tank (doing tank stuff, saving allies and pulling aggro) despite being widely considered replaceable in the current meta (and is, tbh - because there is no tank meta in this game.