In my experience, no one is really duped by these big companies. The people writing the checks generally start their search with magic quadrant leaders and then further narrow it down to the biggest players because "we can't trust our business to some no-name." It's really a situation that companies willingly put themselves in.
The duping starts long before the Oracles/IBMs/Whomever of the world ever reach a customer too - the "analyst" firm role is a huge problem with companies like Gartner/Forester who dream up the "Magic Quadrant" that people purchasing this stuff end up stuck relying on. I've seen in several roles the enormous gulf between what Gartner/Forester will claim about a leading player's capabilities vs the reality, which of course is a state of affairs large Enterprise software companies are all too keen to encourage.
I've never seen a sales consultant answer "no" to a question about capability in an enterprise software sale scenario, ever, across many RFPs. If your job explicitly targets you to bring in x millions of dollars of business a year, you answer "yes, of course" safe in the knowledge that when it does blow up it's someone else's problem and far away from you by that point.
Falling behind? Ship half a feature, lie about it or explain how it's going to be working in the next release (and when that doesn't happen, blame the customer's requests for churn, or the height of the tide, it doesn't matter). In danger of actually completing a contract? Trod hard on the bugs, you don't want that money pump to stop. Larry Ellison needs a new dock for his yacht? Time to increment the product's version field and let a thousand consultants bloom. Ka-ching, baby.
My experience with the space, both as a developer of Enterprisey software at several start-ups and as a customer, was that this corner of the industry is ethically sick, and I won't have anything to do with it.
The people who have to use it, who are probably best informed to point out it’s strengths and weaknesses are usually the last to see it.
A lot of this purchasing is purely "cover your ass" thinking. Going with IBM, Microsoft or Oracle won't get you fired.