Simple.
They don’t want to:
A: be done for handling stolen goods
B: be sued when a dodgy tyre goes pop
C: they make their money on selling the TYRE
My mate works in an independent tyre shop, buys my tyres in (Pirelli’s, 195/50R15’s on a Fiesta, so very common) for £29 each. They retail at £60 upwards and if he sells enough they get a bulk discount on their bill from his supplier. Charges £10 for fitting, which takes up lift/man hours and electric/air for the machines. Fitting is basically at cost. Same for puncture repairs.
So joe public who saunders up with four part-worn tyres gets told no*.
* unless you’re a mate, in which case you get let in after hours, told to do it yourself (easy job when you read the instructions) and told to make him a cuppa after you’ve finished 😆