Cuz this isn't a scripted statement you read off a teleprompter, an incredibly softball interview or even a debate where your opponent is using kid gloves like Biden is used to and comfortable with, this is as an actual hostile human opponent.
A predictable opponent to an extent, sure, but not 100%.
It's very possible for Trump to come out of left field with something Biden didn't fully expect, causing him to trip over himself and maybe not even get back up.
Chris Wallace will probably try to help Joe where he can, but there's only so much he can do.
Trump and Biden have only met once before, during his inauguration in 2017. Compared to Clinton that is super unusual and interesting.
They literally haven't seen each other in person since that day, lol.