Remember, their odds are based on machine runs that crunch all sorts of data (not just national and state poll results, but also economic numbers and other, more-arcane factors) in a formula that changes the closer you get to Election Day. As such, it reflects a lot of the biases of its creators as to which factors are more or less important, when they obviously can't know that.
Also remember that, on the morning of Election Day 2016, Sam Wang at Princeton Election Consortium gave Clinton a "99+%" chance of winning...and that was only because he decided, a few days before the election, that he really shouldn't give her a 100% chance, even though his simulations were indicating her winning every time.
Furthermore, even a 15% chance is only slightly smaller that rolling a pair of dice and having the result come up 7 -- the most-common single possibility in such a roll.
I prefer to look at the poll averages. Those show, at this moment, a double-digit lead for Biden, with around 52% of the vote -- meaning that, in order to win, Trump not only needs to win all the current undecideds, but peel away people who now say they're voting for Biden, which is a near-impossible task this late in a race (assuming we all get out there and vote -- and make sure our votes are counted -- that is).