Because spam in pubkeys make nodes expensive to run. Unspent transaction outputs basically need to fit into ram. I'd rather store the monkey jpeg on 4tb hard drive or delete it if I'm running a pruned node than to buy more RAM to run a node.

For this to be the case, this way of storing monkey jpegs (in pubkeys) has to be more expensive than other options.

So that means there has to be a cheaper way that works.