I mean, it feels strange to me to say that there aren't a lot. I would sooner say that there are fewer rather than that there are strictly few. Just looking at the top 100 anime on Anilist, I'd say that there are actually quite a lot that seem to have primarily adult casts, although I suppose appearances and plot descriptions can be deceiving — as can outright searching by the tag "primarily adult cast".

And then even of anime and manga that normally depict teenagers, there are a handful which have movies or spinoffs or whatever that depict the characters as adults. The YuruCamp Movie from 2022 stands out, that was a great movie. And Lucky Star has now an ongoing spinoff manga called Konata 30 which depicts all the characters in their 30s, that started last year.