Not saying it's the best/only approach but it made sense at the time. For example, Sentry does the same: https://github.com/getsentry/sentry/
Technically in Django you should put all the "static" into a static folder.
Technically in Django you should put all the "static" into a static folder.
If instead you have a separate project diferctory for the UI bits and then just configure it to use the Django's static folder as the output dir (or alternatively, configure Django to to add the UI build folder in STATICFILES_DIRS), you avoid this pain.