I've brought up the same point in a number of threads that have brought up mailing lists over the years. In the specific case of mailing lists like the ones used for Linux kernel and git development, I believe many participants host their own SMTP instances and use them to send and receive messages and have had those set ups for a long time.
If they were to switch to NNTP, they would either have to:
1. Create a new group on usenet (and have to somehow filter out all the spam posted to it or everyone will have to keep their NNTP client kill file up to date)
or
2. They each would have to host their own NNTP server and each one would have to add enough peering servers (incoming and outgoing) to ensure that articles are propogated to them and articles they post are propagated to others
or
3. The maintainer would have to set up a authoritative NNTP server and allow read access, but then have to add accounts for those who need write access (in order to allow them to submit patches).
Continuing to use SMTP allows them to essentially choose option 2, but not have to set up peering servers since DNS MX records handle propagating the emails they send.
That said, having a NNTP mirror of a mailing list like gmane[1] and public-inbox[2] provides the best of both worlds. You get the benefits of NNTP for reading the mailing list, but get the benefits of propagation via SMTP.
[1] https://gmane.io/
[2] https://public-inbox.org/README.html