Never been in such a situation, though I can imagine:
- Remaining cabin luggage is "offloaded" by low-paid workers.
- This stuff is brought "somewhere", where the authorities will scrutinize its contents.
- The stuff is relocated to a location to be united with the passengers, still being around.
- Passengers will need to identify which items are theirs, either verbally or by pointing.
- Of course, when collection/returns are done rigorously, there will be some cross-references with allocated seat and location retrieved (read: time-consuming).

So, yeah, quite some moments when those in need of nice things or money, can grab their chance to have a free shopping opportunity.
Not to say, all those luggage items that don't have suitable identifying marks will be a challenge to get that back.

And this whole process will of course take a huge amount of time.

So, yeah, there are pretty valid reasons passengers don't want to be separated from their valuables/possessions.

Note: Many people having emergency vacated aircraft "with" hand luggage afterward reported, it was not theirs, though they took it with them, because it blocked the evacuation. So (maybe even by far) not everybody seen evacuating with hand luggage is saving their own stuff......