Because we expect a lot of hand-holding.
You deliver a product. It works, but wait: there is a problem. Back to the vendor: we need to know how config X.
Now, it is not that I'm not capable of reading up on how to, say, edit $APP_ENGINE to be secure. I have a development environment, I can play around with this setting or that. I _can_.
I don't have _time_ for that.
I've got my normal workload, I've got projects, I've got a stable of applications to configure, babysit, fret over. If I had to spend my time re-discovering how to make each and every one of those applications work, that is all I would be doing.
No. What I need from my vendor is a cook book approach: if you have Solaris, do this. Redhat, do that. Windows, the other thing. I need guidance, so I can hand the book to a junior SA and say 'follow these directions'.
If you, the vendor, can't provide at least that level of hand-holding ... it's going to be a rocky relationship.
Thats 90% of battle won. Problem is most "startups" don't even get to that point. The reasons are not as simple as just "I dont have time". Enterprise contracts are usually for many years and vendors have a very strong hold on enterprise clients. I support a multi million dollar vendor system at my employer and I can tell you even though most of the client "users" hate it, no one can even think about getting rid of it.