> Many of these boxes may not even have the spare CPU power ...

I don't think that's generally how these networks use residential exit proxies. There are at least a dozen well-developed frameworks out there for decoupling the crawler from the network exit point.

Most res proxy exits are just slinging bytes for clients using SOCKS, or another tunneling protocol.

If nothing else, a modern scraper will want better control over their TLS fingerprints, and you can't get that if you're depending on the on-device TLS libraries alone.