Why some (ok, a lot of) sales and buying guys are there just to transform any service experience in something bad?

In my experience, most sales guys promises anything, with absurd deadlines and it's up to you to make it work. Things never get good enough and, after all, the client doesn't like the work, you don't like and the sales guy doesn't like you could not make a perfectly task.

Buying guys are even worse when buying services. Email 30 developers in BCC: "Hi, I need this done by tomorrow morning and I would like to know how much you charge for it. I need your proposals in the next 30 minutes". He closes the deal with the fastest and cheapest proposal.

These things make absolutely no sense to me.