Political gambling sites aren't really banking on rube bets the way that regular sports gambling sites are banking on Dallas Cowboys bandwagonism. Different audience.
The issue is not that Trump has the best odds. Of course he does as the presumptive nominee and incumbent. It is that he has dramatically better odds than any Dem and even more importantly, he is pretty much an even money bet against the field. The fucker is corrupt, nearly his whole administration has been shown to be incompetent and/or has left in shame, the Democrats have been overperforming in both the midterms and one-off elections, and he is going through an impeachment process! Still even money. That is an indicator that the odds makers don't think the Dems have their shit together (accurate) and that the front runners for the nomination all have obvious, easy-marketing flaws (also accurate, as RJ has consistently pointed out in this thread). If the economy is still churning and one of the four front runners is the D nominee, Trump will be an overwhelming favorite next fall. And that is pitiful.
Praying that one of the late entries gets serious momentum quickly. Unlikely, however.