You're referring to BCP-38, aka RFC 2827. It's actually decently hard for a transit network to do this at scale. You can do it when you're a small ISP, but the administrative stuff becomes harder as you get larger and are constantly getting more allocations, buy out other ISP's, etc.

Every content network should do it. Not a huge win there, but it's something.

I'm not saying we shouldn't try, but there are countless, very long threads on NANOG about why some transit networks just can't do it.