The knowledgable end user [or his equally knowledgable rep] writes the spec. Bear in mind that this exercise alone can take a considerable amount of time.
Then it is staffed up the chain to the PE. That's where the problems start. People with little or no recent operational knowledge start chopping bits out to save money. Despite front end protests, the PE decisions hold sway and the user ends up with a compromise.
I saw this process in operation twice in a major way. The worst was when the front line spoke to the only manufacturer during the writing process to ascertain what was available and workable. The manufacturer, on receipt of the revised PE spec, rang the guy who wrote the original to be told that it wouldn't work. PE was adamant that it would [the manufacturer didn't know what he was talking about and was trying to diddle us - I kid you not!].
Said system was installed at a cost of £X squillion - when we fired it up for the first time in the ac the system promptly crashed and the fix cost £X squillion extra. Total cost = £original + lots more besides.
PE don't like buying off the shelf because otherwise they would have difficulty justifying their posts.