Because in my personal opinion, if you're not taking those tanks and immediately diving them, they should be reanalyzed. Especially trimix. I fill my own gas and it's analuzed 3 times. Once coming out of the compressor, once out of the full tanks once filled, and then a final time before a dive (either before going in the truck or at the site). Get a pp filled trimix tank from a shop and analyze it within an hour of them filling it, then reanalyze in a couple days. I bet there's going to be a slight or major difference.
Personally I believe all divers should test their gas be it air through trimix for both % content and CO prior to every dive no matter when the tank was filled. If you can use a shop analyzer right before the dive, great. If not buy your own.