“Nonlegendary planeswalkers don’t even make any flavor sense.” Planeswalkers on a card didn’t make any flavour sense either. Didn’t stop you then. Plus, the legend rule changes has weakened legendary to such an extent that any given game is likely to run into a situation that makes no flavour sense anyway. ,
Planeswalkers are coming to aid you, much like the creatures you summon. The flavor’s fine. If that answer doesn’t satisfy you, non-legendary planeswalkers are also a bad idea for play design and branding reasons.