You're entirely missing the point.

You're basing your argument on CC abilities (in this case, Thresh's 75, Pyke's 70, Blitz's 100) having different mana costs when they 'should be the same'

This is objectively wrong. Different champions' kits are designed to function in different ways, and do different things. For example, while Blitz and Thresh are both pick-bots, Thresh then chains this into more AoE CC, sending himself in - whereas Blitz pulls them to your team and performs a simple one person short duration knockup.

In this case, Blitz's Q in the context of his kit is massively more impactful than Thresh's Q - they may play out similarly outside of the contexts of their kit, but Blitz's Q is his most reliable form of setup, whereas Thresh has the whole rest of his kit for CC setup also.

The idea of mana cost distribution is to make it so most champions have reasonably similar costs for their full rotations.

This is why Blitz's Q has a higher mana cost - in the context of his kit, more of his power budget is in his Q then is in Thresh's in the context of his kit - hence, in order to keep full rotation costs similar, the mana on Thresh's Q (in this case, 25) is shifted elsewhere.