I think the problem is that there isn't an ideal solution. Not many kits come complete, a lot of times you're on your own for recovery, motor retention, etc. And even those that do come complete often require adding epoxy. So short of a fully-assembled (truly ready-to-fly) kit any given model would either be lacking detail or based on the build assumptions of the person who created it. So when grabbing somebody else's model you're probably either getting the base kit (which I'd say isn't terribly useful as it will be light, etc), or else it includes somebody else's choice of the extra hardware (which arguably isn't a bad thing, it could offer some guidance on a good chute size, etc, but you may decide to do things differently). I got tripped-up by this when I grabbed a model from Rocketry Warehouse's page, it represented the rocket as-shipped, but you certainly couldn't just throw a motor in it and get an accurate sim.
I always try to create the most accurate model I can, weighing every piece, how much epoxy I add, weight changes due to painting, etc. So I get accurate Cp and Cg predictions on my models, and the sim estimates have come within the +/- 20% motor tolerances (assuming an early/late motor eject didn't significantly impact things). But I also don't tend to post these files on RocketReviews because they are very specific to how I built my rocket (or in some cases how I re-built it after flight mishaps). Useful to me, but others may have to do a lot of work to make it useful to them as I may not even have the stock airframe length anymore after zippers and the like.
I think there can also be a lot of variation or other things to consider as well. For instance when I made the model for my Excel w/DD, there were 3 airframe sections, the upper and lower BTs and the vent band. While they were all the same ID/OD, I had to define 3 different custom materials to get the proper weights (I preferred to do that vs. overriding the mass). The vent band had a different tint so it seemed to be made differently (or from a different batch), the upper and lower tubes looked like they were cut from the same stock, but the slots in the lower tube caused it to be lighter (so a lower unit weight, and of course a Cg offset that came with those slots as well). But had I picked a single material for all 3 pieces either all 3 would have been off (none of my unit weights matched any of the ones provided in OR's defaults) or else at least 2 of them would have been off.
Getting a reasonable Cp (ignoring other tool complexities like fins on boat-tails that only some people seem to know how to implement) probably isn't that hard, but if people are not even getting tube lengths right then that obviously doesn't help things. But getting the weight and Cg accurate takes a lot more work, and probably isn't right for any rocket other than the specific one the measurements were taken from.
