Many corp email smtp servers will IP block your email server (big thanks to spamhaus), or won't support no-DNS email address and servers (which is RFC from the start) or won't have the decency to handle grey listing or will send all your emails to their spam boxes (gogol) even though ppl did remove your emails from their spambox.
IAPs won't provide a stable public IPv4 address or IPv6 prefix. UPNP NAT port redirection (IPv4) will have bugs on the IAP router/modem.
Buying a DNS and configuring a domain is a pain. So few DNS registrars support automatic domain configuration via the standard dynDNS protocol (is this even a thing?).
The self-hosting devices on user domestic LANs will be pown by very "smart" hackers pushing those very users towards big tech (I wonder who is pay... pushing such hackers to do that...).
The path of least resistance will win, always, even if it means giving way to much power to some corps:
Lambda users _will use_ comfy centralized services mostly, and those centralized services, once big, will try to zap away any alternatives or interop (which most used in the first place to get there).
Like lambda users _will use only_ the pre-installed OS on the computer (or mobile phone) they bought, same idea.
I am talking about nearly everybody else who is not "us", the 0.1% (ironical).