Because one implementation of coinjoining needs 4 more bytes of data (which you can set in your node BTW unless you suddenly think filters work) that's why OP_RETURN needs 100KB?
Discussion
Do you want everyone to run knots filters?
If not you don't want anything about bitcoin to change you are just selfish and externalizing costs and risks demanding I do it for you.
If so, you would ban whirlpool.
What I want is irrelevant. What I do is the matter. Also, nothing about whirlpool REQUIRES 46 bytes. They are just not good at using metadata hashes efficiently.