Because it is removing a limit that spammers will not use anyway because inscription are cheaper.
Discussion
That's not how Bitcoin works in practice. If something is possible some people will do it even if it doesn't seem rational.
If we lift the 80 byte limit on OP_RETURN the most likely outcome is that we'll start seeing plenty of huge OP_RETURNs in the mempool in addition to the current inscription spam.
It’s even more critical than I thought.