Just out of curiosity, are there still gambling sites that didn't settle the bet even there is a clear winner already? Fiat and crypto-gambling site?
Post-election investigations should not be included as a factor in these bets.
Or they are just waiting until Biden officially assumes office next month, January 2021?
Yes, there are. Some bets were also created weeks after the election, some don't necessarily involve honest winning of the election, but otherwise 'winning', some are about the upcoming electoral session's vote counting, or the inauguration, etc.
What's their excuse? And can't believe other bettors chose a not popular site and entrust their funds there for long?
Often it comes up to the wording of the bet or its initially drawn up decision logic.