A lot of people do post traditional comics, the thing that happens is they take the time to polish their art, be it by scanning it instead of taking a photo with their phones, do digital lettering, or do minimal digital fixes. Or combine it with digital techniques, there are comics you would never know weren't done 100% digitally.
What happens is that in most webcomic platforms, colored comics tend to do better than black and white ones, and since making and materials tend to take time or be expensive, many artists go for digital since its cheaper/ faster even if manga is popular. Formatting is also important, you can scan your traditional pages, but arrange them for Vertical format in order to make it fit more with what the userbase wants.
There is no prohibition nor rejection regarding traditional comics, you can totally do it and do it as you please, just is important to know if you seek for popularity or engagement, then... readers do have their own preferences too.
