Most people are terrible at OPSEC. If your online persona is used long enough and garners enough attention your real identity will almost certainly become known (extreme example: Ross Ubricht).

This is compounded by the fact that people tend to say things they wouldn't otherwise say if they knew they weren't anonymous.

The other issue is it's harder to capitalize on the reputation of online personas in the real world, if you care about that sort of thing.

If you don't care about reputation online or offline then just generating new identities on a regular basis could work well.