Why do people hate vendors

So I have a situation where I not only provide services as a vendor but consume services from other vendors. I have always tried to be nice to the vendors I work, with but noticed that in many cases others do not extend the same good behavior.  This is just not to me working as a vendor but I see it all the time with companies working with other vendors.

Some of the top request that I see from companies working with vendors:

• Unrealistic timelines to provide functionality that they will not even want for months after it is completed
• Just plain being rude to the vendor.
• Using promise carrots to get something they want but not following through on the promise.

I think if you do not want help from a vendor just tell them straight out.  Do not lead them on they have better things to do with their time then to try for work that they can not get.