Because I've studied politics for over 40 years. You can go back and look at where candidates spent their time and see what their internal polls are telling them. Right now the public polls are saying Biden is winning in almost every single battle ground state. IF this were true then Biden would swing by PA, WI, and MI just to not do what Clinton did in 16'. She didn't go there at all. He's living there. If Florida was close why would Biden not go there? Georgia? I can tell what their internal polls are saying by where they are and where they aren't. Why is Biden not in Ohio or Iowa? He'd never spend this much time in the rust belt if he were up big there. It makes no sense. His internal polls are showing him in trouble in the rust belt or else he'd be somewhere else trying to pick off other states that have him with a small lead.
Maybe they are trying to not make the same mistakes Hillary did. Florida is the most important state for Trump and I believe it is very close. Biden can win w/o Florida while the chances of Trump winning w/o it are basically 0. That being said whoever has won Florida has won the election since 1996 so both campaigns should be focusing on it