I think the much better question is why would somebody use OP_RETURN for stamps, BRC-20 and Counterparty. It does make a difference for some of these protocols if the data is in an output, or the scriptsig, or witness part of a transaction.
Discussion
With stamps the explicit goal was to bloat the UTXO set. Their marketing framed that as "unprunable".
Counterparty was designed before SegWit and due to fee spikes around that time was pretty much unmaintained afaik.
BRC-20 is just crazy. JK, I have no idea how that one works.
Stamps use CounterParty under the hood, which simply switches to bare multisig once the payload goes over 80 bytes. With the proposed Bitcoin Core change to policy, or just considering relay reality, CounterParty could stop doing that. But afaik nobody is maintaining it, and the inefficiency is a feature for some of its users (stamps).
You're right, those are in another class altogether. Upvoted your post and answer fwiw :)
