Because if you have an accident and they find out you had tyres/rims different from stock they will absolutely either refuse to pay out the claim or reduce it significantly. Especially if you put on tyres of a different size than that approved by the manfucaturer, they can very easily say that a tyre that is too narrow/too wide affects how the car handles and it would have contributed to the accident.
And then you take it to court, and you get a mechanical engineer and a racing instructor to say that a 215/45R60 is not going to cause an accident that a 205/45R60 wouldn't have caused, since tread wear, hardness, speed rating, structural design, and road conditions all are the most important factors in traction. Identical tires with a 10mm width difference will perform nearly identically, but the same size tire using different manufacturing processes will result in completely different traction. Tires of the same size rating but different manufacture can have comparatively narrower or wider (due to sidewall stiffness) or flatter or bulging (tread patterns) geometries.
But my bet is it's more a matter of following a contract than whether it actually caused an issue; either way I can see not wanting to deal with the potential hassle.