The parties are so far apart right now that it’s hard to imagine somebody who doesn’t at least have a strong preference. I think many undecideds are Republicans who don’t like Trump and Democrats who don’t like Biden. The decision isn’t “vote Trump or vote Biden”. It’s “to vote or not to vote”.
To use myself as an example, I didn’t vote in 2016 because I didn’t like either major candidate. But despite still not liking either major candidate, I’m voting this year… even though it’s largely just for my conscience, since my state is usually decided by double-digits.