Because it's all about how much health they have and their speed.

Is that klee in your pfp!!! From genshin 😭 

I think it is about their base stats, and not their ability alone. Without their ability active, these mechs don't have high speed or support role. But their base stats like weapon carrying capacity, base speed and HP are what is key in determining the base category of the emchs. Otherwise scouts are also attackers, attackers and tanks are also support, support are also tanks, attackers or scouts. But that's just the variety and uniqueness of each mech, like being a support attacker, a tank support, an attacker support, or a tank with scout ability like Death walker and Vortex. Mech Arena developers are very particular that a mech should fit well and should be suitable to the role and category for which it has been created and Pasha has said this Publicaly with emphasis. 

I just don't get why a tank is outspeeding the mechs with the highest base speed under any circumstance. On top of just coming back to life if you kill it while its ability is activ.e and having high energy capacity.

There are mechs that blend categories and then there's mechs that do everything to a degree that makes it so it doesn't matter they do everything worse than the best. Jack of all trades, master of none, but good enough to get the job done. That's one way to make something truly overpowered. Deathwalker feels like that. But you have outlaw running around now. Thankfully most people hold on to that for deathwalker but it's still annoying. Especially when everything is given a 50% reduced ability cooldown. BTW, not everything should get a 50% reduced ability cooldown.

oh hell nah solis and salvor energy make no sense at all, while seeker has a hp, energy cap, skill as a scout but they choose to place it as attacker.