This uniformity benefits users in several other ways. Consider looking for an systems person. If all computers had the same interface, it would be easier to find and judge candidates for the job. The pool of applicants would be bigger. Basically economies of scale mean that a large producer can either do it better or cheaper than a small producer, other things being equal.
Software is revised frequently, and it very brittle.
This is unfortunately a self fullfilling belief. Since software is brittle everyone knows they may be hung out to drive when a smaller company folds or discontinues a product. Oddly enough this leads to users buying from the largest vendors. The funny part is that these vendors set the standards for correctness. Probably the first company to stop writing software for 16-bit Windows was Microsoft.