It’s perhaps not as simple to implement in scoring software as it is in other MIDI-based software because of the indirect relationship between the notation and the resulting playback: with MIDI, one event is always one event, but with notation you have to convert each printed notation into potentially many MIDI events, and then if you want to loop in the middle of something that produces a lot of MIDI events (like a trill or a tremolo) then that needs special handling.
All that said, I definitely think that having a proper looping feature in Dorico would be useful, and it’s on our wish list for the future.