Because traditionally they were served "courses". However, it's all biryani in the belly. I think the old ways of serving a main with sides is still a tip to that era when it was plated separately. I'm ok with having it all mixed together so long as the flavors complement. I don't want a steak, cubed, mixed with my veggies and mashed potatoes, all in a bowl. Some things should be separate, some things are ok being together/combined. Western style cooking has been dominated by French Cuisine and traditional Countryside cooking. Only within the last 30-50 years have they started branching out into Asian or South American dishes.

Its way more approachable today than it ever has been with YouTube videos, online blog recipes, availability of supplies.