I know somebody also mentioned this but why does this exist

Probably to keep people from making alts and spamming people

Makes sense

It's to prevent spam bots

and that

1. Lowering the risk of spam.
2. (My theory) Imgflip is an entirely bootstrapped company. This means that the funds coming are from a limited source (+ads). Running their servers on this vast array of users is really expensive. So limiting extra comments from new users (and potentially alts) will reduce the expenses of Imgflip's servers.
3. Reducing the power of alts.

4. Was introduced after the s3xb0t attack

I think