He keeps on saying it, but he's not screaming CEASEFIRE NOW! so those people will continue to ignore it.
He could literally scream "ceasefire now," but because this cause has morphed into "Israel is perpetrating genocide and needs to face the consequences for it," it would mean nothing to his critics on this issue.
He could scream "ceasefire now" with the crowd and some of the ceasefire now crew would still find a reason to criticize him. As someone else mentioned, most of these people don't like Biden and are using this as an excuse to project their hatred of him. There are nefarious actors as well, but I think that at the end of the day most of these people have good intentions but are misinformed about the situation. Biden should remember that there are more "ceasefire now" people who'll vote for him in 2024 than there are people who'd abandon him if he does a 180 on his current policy, especially since Trump is in support of Israel as well.