[acdha]

ISPs used to do that more commonly before HTTPS killed it but it's an expensive service to operate: very high traffic and if anything goes wrong your customers have a bad experience on the entire internet. The only way to do it is by intercepting TCP connections to port 80 so that system has to be as close to 100% uptime as you can manage.

Site owners generally hated it, too, since tampering proxies were a perennial source of compatibility bugs and protocol violations even before you had things like the ones which tried to “optimize” images by recompressing them, giving everyone on that ISP a bad experience which you don't know about. Stack Exchange has a number of threads where someone was trying to figure out why only some customers had complained months-stale content (Hi, Telemundo!), low-quality images (Hi again, Telemundo!), mismatched languages or truncated/corrupted contents, etc.

[acdha]

Ah, it’s been too long: everywhere it says “Telemundo” above I meant “Telefonica”