Spam reduction from botnetted machines, and selling business lines, are the reasons I have seen given.

Major reason will always be support burden. For http to work reliably you need to put clients into DMZ anyway. If you let computer illiterate end users operate in DMZ you will end up supporting them anyway, because in the your network will be at risk of depeering/blacklisting. Supporting http behind NAT is again significant support burden that costs actual money while your competitors offer service access cheaper. It makes sense to not only not support http behind nat but block ports altogether for retail and offer proper liability waived DMZ access under enterprise plans.

ISPs are media companies. They do not want regular people publishing content or having a voice unless they can get their commission. The world would not have become just 5 websites with identical content if everybody had a server at their house.

Not sure but probably some security thing - if you don't need it why leave it open to abuse.

An ISP I've used in the past hasn't allowed self hosting on a standard plan but has allowed on a plan you need to ask for specifically.