Two reasons. First, there’s GOP just blanket hating on anything without their party’s Seal of Approval, sometime posing as democrats.
Second, it’s in the nature of progressives to want something better. To point out that, while [thing] may be OK, it could be improved, even in small ways. So, while you’re comparing Biden and Trump, they’re comparing Biden with some Platonic ideal President. Most of them will, when it comes to the actual ballot, and they have to choose between the two actual candidates, vote against Trump, but they’ll grouse about it. The others probably weren’t going to get off the couch for anyone, anyway.