Because anyone but Trump is 'controlled opposition' or 'establishment'; actual policy and actions taken matter less than appearing as a realistic challenger to Trump, thus DeSantis has to be part of 'the enemy' in one way or another, to those who still idolize Trump/mistakenly think Trump running is something the elite don't want.
The Dems in DC and Biden are scared shitless of of DeSantis, because he'll actually do the follow through Trump didn't do, doesn't have the same ego or baggage as Trump, and has a proven track-record in Florida to bring with him.
Biden and co would love to face Trump again, because the 10% of the population who are true independents are the ones who most often swing the key races, and Trump turns them off more than Biden does due to Trump's bombastic personality.
2024 is a very likely Biden win, even without electoral fuckery, and Trump is not without blame for how he turns off the swing voters.