Flip-side, an honest & competent purchaser can use "cooked" RFP's to try to weed out incompetent, indifferently-honest, cost-overrun-prone, etc. would-be suppliers, when they're forced to follow a "publish RFP and take bids" rulebook.
Yes this is also an additional gotcha that makes this process an even deeper quagmire.
I don't quite understand
Any sales process large enough to require an RFP will attract low-cost bidders who write good proposals but consistently fail to deliver quality work. Writing an RFP that's specific to a single company is one way to avoid dealing with the low-cost bidders.