It's largely driven by the people who buy the software having a complex business need that they want to fulfil, but who don't know the best solution, if it even exists.

So they surround themselves with advisors (consultants, project managers, architects, etc), some of them good, some of them bad, and between the 3 groups (the buyers, vendors, and advisors), they try to come to an agreement where all 3 are happy.

The sales process is basically nudging all 3 groups into positions where everyone is happy, and can be a long and painful process.