It does seem bizarre I agree, but both him and his dad (Director of Sports Medicine and Performance for the Golden State Warriors) have come out recently and said they wouldn't mind him returning to BU, so I really don't know at this point. It's in an Athletic article but Rick said:
"Me and (Warriors coach) Steve Kerr talk about this all the time. There's a value to being patient and developing at a level that's more conducive to development. The NHL and NBA aren't developmental leagues. You're either ready or you're not. But college hockey is more conducive to that with the schedule, the work in the weight room and practice schedule. An Arizona scout called me and goes, 'For what you do for a living, where would you say he is on his developmental curve?' Honestly, he's a puppy. He shaves once every couple weeks. He's not even through puberty. He really is a kid, kid. He's got a little bit of maturing and growing to do, just getting that thickness of a man's body."
Playing in the NHL as an 18 year old is tough. Bedard is Bedard and he had no other choice either way, coming from the CHL. Then there's Benson, who's an outlier. Fantilli could have stayed at Michigan for another year if he wanted to, but he's 20 months older than Celebrini.
When we look back at players like Jack Hughes, it might have been beneficial for him to play college hockey for a year instead of going straight to the NHL. It'll be interesting to see what Celebrini decides to do. He did have shoulder surgery last summer, so I wonder if that will play into their decision as well, in terms of wanting to take more time to physically develop and get stronger.
EDIT: I also wonder if he goes to the NHL right away, if whichever team wins the lottery on Tuesday goes the Leo Carlsson load-management route for him. Something to consider.