After the first election day, Trump was up on predictit ~60/40. This tells me that the betters have no idea what they're doing. While Trump was leading in swing states, most metropolitan areas only counted a small fraction of ballots, so the early votes had very little statistical value.
Also, there are often tiny arbitrages where buying "no" for Biden would cost a few cents less than buying "yes" for Trump. In theory, you can make a guaranteed 1% return on betting no for both Trump and Biden, and it was as high as 5%. You can't take advantage of this arb due to fees on predictit, but you'd think rational investors would at least try to keep this even.
Rational bettors are trying to keep it even. Why would they donate money to predictit just to reduce the spread? Predictit’s fees reduce the efficiency possible in their markets.
Also, there are often tiny arbitrages where buying "no" for Biden would cost a few cents less than buying "yes" for Trump. In theory, you can make a guaranteed 1% return on betting no for both Trump and Biden, and it was as high as 5%. You can't take advantage of this arb due to fees on predictit, but you'd think rational investors would at least try to keep this even.