Actually, there are lots of reasons. Not all material needs to be encrypted, because not all material is privacy sensitive. Which means, the only thing left to deal with is tampering.
A better alternative than blocking it altogether - because that mostly renders transparent proxies useless - is to force links to HTTP content in HTTPS transferred sites to be accompanied by a new attribute in the anchor tag with a file signature (or a link to an also HTTPS-only file containing the signatures) that could be checked by the receiver. That way, no matter where the content came from, if the signature matched, the content would be the original. We have tried to push that idea. And yes, we know there are some caveats.
This is important because the performance of HTTPS compared with HTTP, excluding any proxies, is substantially different. In tests we made, a RPi4 could easily reach 10000 transfers/second in HTTP, but only ~250 transfers/second HTTPS (local network file, overprovisioned testing endpoint, same file for HTTP and HTTPS).