Some make serious money out of scope creep in IT projects. Sometimes I think that this must be how the major consultancies are sustaining themselves.
Someone wrote that the /average/ IT project ran 10x over budget. This would be completely unacceptable in any domain but IT.
All but #1 have a structural incentive to make projects bigger the same way a mechanic has a structural incentive to "find new problems." The client always has final say, but people underestimate how hard it is to keep saying "no" to multiple reputable firms all warning you against going too small.
That said, this kind of swindling is far from ubiquitous. There's a philosophy that blindly upping scope is shortsighted, and you do better in the long run by actually representing your client's best interest. Where I worked, I found it pretty easy to surround myself with people who thought this way.