That quirk is true on PredictIt, but currently on Betfair there is a very liquid market with millions of dollars on both sides where the implied odds are roughly 95% for Biden and 5% for Trump.

Now, betting markets do have the problem where even if the bet was a "sure thing", it might not be worth betting for a ~5% gain when you account for the risk and effort related to getting the money into and out of the betting market, and I think that partially explains why the market has settled where it is. But still, this highly liquid market had Trump a pretty strong favorite on election night as well.