1) When someone buys custom built hardware, they know what they want, and the specifications for it. When someone buys software, they want "something that does xyz, is just like facebook and paypal, but also has our logo." And then it turns out that the problem they're trying to solve is completely unrelated to any of those things. But you can just rewrite it to do those things too, right?
2) If there's a problem with the hardware, the customer that paid the money for custom chips will notice, and will have the technical skill to lay the blame on the vendor accurately. This means hardware manufacturers need to deliver quality, and since point #1 guarantees they actually have a specification, they can therefore engineer to it. In software, often the client takes your application, puts it on machines from the mid 90s that it won't run on, gives it to minimum wage employees to use (so management may not really care if it works), and then ask if its too late for the product to also be an app on their iphone. When they can't do anything with it 3 years later because it was written by people who have never looked at a computer before, the customer will just say "software always has bugs / always going out of date, find a new consulting company to write us a new one", as if its normal to have to build a completely new system to do the same thing over and over again every three years.
