The issue is that domains are cheap and plentiful, and a spammer can easily claim to be sending mailflow on behalf of let's say, 1000 users, each sending 5-10 emails a day, and thus, a single IP "forwarding" (sending) mailflow "on behalf" of 10k domains, with 1k users each, with each user sending just 10 emails a day - would result in a pretty nice batch of spam making it through.

As things currently stand, the only "expensive" resource that is hard to acquire or fake somehow is source IPs with good reputation. Fair? I'm not saying it is. It's just effective.