Because there are externalities to segwit and taproot that allow people to create transactions stuffed with relatively large amounts of arbitrary data (like jpegs). Since many of these people have no intention of spending from these transaction outputs, this poses a long-term risk to nodes that have to use their resources to keep track of these UTXO's forever.
Boosting the OP_RETURN limit is a limp-wristed minor rebalancing of the incentives that allow people to store arbitrary data in bitcoin in a way that is less of a resource drain on the nodes that run the network. It likely won't work, but at worst, we have the same problem we had before they raised the default limit.
Core doesnt know how to solve spam, and it looks like they're content to quietly admit to themselves they broke bitcoin a little trying to scale. They'll never admit it publicly though.
And knots does the know how to solve it either, but that won't stop them from saying they can, if only they could overwhelm 85-95% of the network, which they never will.
