I'm not sure what the actual reasons are, but I can imagine people aren't very eager to invest time replicating all the work that went into Apache or Nginx by implementing the same in Python.
Those existing solutions are well-tested, feature-rich and fast. Just use them. It'd be a massive waste of time for every web framework to ship it's own web server.
The Django quote you gave tries to highlight that a web framework and a webserver are different things and that they don't want to include the later in the former.
> We’re in the business of making Web frameworks, not Web servers
I'm not sure why Go didn't go with that approach, because I'm not an active Go user. Does it have good https support? Load balancing? IPv6? Static file caching?
One reason that it is more desirable to offload web server work in Python than in Go is, of course, that Go is typically faster. In Python you'd not only losing a lot of development time, but also performance. In Go it's just the time.