Can anybody explain why polymarket and predictit sustain such different prices? Finding these differences wildly implausible.

Design and software at Apple. Also into AI, philosophy of mind, ethics.

because prediction markets aren’t actually accurate historically speaking

Actually prediction markets have been shown to be quite accurate, at least as good as expert pollsters.

Section Section 2.4 here for links to studies:

https://www.astralcodexten.com/p/prediction-market-faq?open=false#%C2%A7why-believe-prediction-markets-are-accurate