Why is it that the Bay Area Chevy and gmc dealers don’t want to do the trans oil flush let alone other tsb, PIT and nhtsb recommended repairs?

we have a 15i Yukon Denali xl and have had every typical issue except the buffering. When the ac condenser broke we had to take it to Dublin gmc to have it replaced because the local dealer wouldn’t do it.
the local dealer had our Denali for 3 days on two seperate occasions and “couldn’t replicate the shudder in transmission” yet never put any miles on it.

the bearing in the steering wheel causing the creak and pop... unable to replicate...

now when it’s out of warranty all of a sudden all of these issues are apparent to the tech... but they can’t find record of me bringing the vehicle in to the dealer during warranty time for those issues...

drive the truck an hour and a half to the Dublin gmc to see if they will see if gm will cover it still.

29,xxx miles on the truck...

We have been considering trading it in on a new yukon xl Denali with the new rear end etc. but with the problems we have with dealers honoring the warranty, we may go to a Lincoln...

I called gmc two years ago and the answer was “we trust our dealer network”... but had no answer to why the dealer hadn’t updated the file with all the details...