Because if you continue hanging out with that person, the feelings will always remain. Like I said, maybe they somehow changed, but honestly, will a guy (this example is hetero-cis obv) who was initially attracted to a girl and then told "hey let's just be friends" actually change his feelings? I mean they're feelings because we kind of can't control them. So the guy needs to be honest with himself and with the girl. Otherwise he's basically staying in there to "try and get in" at a later time. That's disingenuous. He's better off spending his time trying with someone else. Maybe in that case he can remain friends with the girl since he has other options.

Interesting, can you expand on this and why you shouldn’t be friends with someone you’re attracted to? Like, do you think ‘platonic’ and ‘romantic’ are binary conditions?

Not strictly but they are more cut and dry then people like to think. And it seems to be perceived differently along gender lines (at least for straight cis relationships). Check out this video and look at the responses: https://youtu.be/T_lh5fR4DMA?feature=shared.

Now whether these responses are socially conditioned or somehow "innate" is a debate I leave to the scientists and sociologists, but there is obviously a perceived difference. I mean just look up "friend zone" standup routines on youtube or anywhere really, people talk about it all the time (https://youtube.com/shorts/zbjMJBixZI8?feature=shared and https://youtu.be/_KE6Y3VrMg4?feature=shared). The truth is that there are many dudes basically remaining in "friendships" disingenuously in order to get with the girl. This is not only disingenuous, but if the guy harbors resentment on being only considered "a friend" then it can lead to violent outbursts later in the "friendship." So it's not only bad for the guys involved, but can be potentially life-threatening for the women. So like I said, it's better to admit how you feel and if you still have those feelings, it's probably better to maybe be aquiantances at best, but not friends (unless your feelings have actually changed, which I think represents a tiny, tiny, tiny fraction of those who get friendzoned).