They could but the fact remains that Trump has agree to debate on networks and with moderators who hate him and wish to destroy him.
His calculus may be something like this: 'Virtually everyone who watches OAN, RSB and Real America's Voice will vote for Trump. There may be Independents who will watch the CNN and ABC debates but not the conservative outlets. These Independents and even some democrat watchers may be positively influenced by Trump's performance.'
IMHO - Although Trump's debating on TDS networks may be well intended, by agreeing to these debates he foolishly allows TDS networks, TDS moderators and post-debate TDS network opinion(s) to frame him. He's taking an unnecessary risk and gamble. At the very least, why didn't Trump insist on having non-TDS moderators in all debates?
I remember when Trump was elected in 2016, I thought that when Sean Spicer was the press secretary that Sean would only take questions from conservative outlets. That was a good idea but never lasted if even implemented at all and they should have. This was planned as Trump was always railing on how biased the fake news media was portraying his rallies, so it was understood he was going to fight back by not taking the fake news outlets questions at the daily press conference, and at the least revoke the press pass for the obvious TDS reporters (Acosta).