It's probably because when faced between paying X for continuous support (where X is a big number) and "we will keep around a few computers for when it's a problem in 20 years", most companies are going to go with the later.

Even if it would make financial sense long term, how likely is it that the person making the decision will be there in 20 years to deal with the mess? On the other hand, it will look much better on their next quarter results.