I think the removal for packing is a big practical problem.

Marketing wise, when you integrate the rack, you have to sell the rack and bike combo as one. A guy might not like your rack, but like your frame, etc... But if the rack is part of the frame you are stuck with it. A lot of people seem to react positively to the fact that there is an integrated rack, it seems hard core, but do they actually like the result? Most cases they probably would buy it if only it was a little bit of x or y, and cost the same as a rackless bike.

Another problem is that while I don't agree that standard points are all you need, an upgrade is clearly advisable if not necessary, but most integrated racks seem overbuilt. In essence you are sharing structure, but the integration is often there with heavier tubing that the bolt together racks.

This English bike uses small tubing for the whole thing:



The Hibell rack moreso than integrated is a "your-stuck-with-it-because-I-welded-it-on" thing. No load sharing or particular efficiency there.