Edit: Once while sitting on the private sector side of the table (yes RFPs were often drafted in consultation with the company that would eventually win them, if just informally), a [state] government employee expressed concern over the wording of a particular requirement because he was worried that it would open the door to our primary competitor being qualified to fulfill the bid.
RFI process is usually far more open.
Having said that, just as often, it's due to cronyism.
The line between cronymism and efficiency is in the eye of the beholder.
It's pretty clearly an example of corruption; the entire purpose of single-sourcing a contract is to defeat the RFP, or bidding-out, process. A decision is being made by someone who isn't supposed to have the authority to make it.
Did you type this on the cheapest thing capable of making HTTP requests?
Do you live in the cheapest structure capable of maintaining its interior at a non-life-threatening temperature?
I submit that lowest-bidder rules are patently absurd. People who have to deal with something day in and day out ought to have the authority to select a reasonable one.
Rigging a bid in a way that compromises the real task at hand in order to personally enrich yourself is clearly an example of corruption. Rigging a bid in (what you think is) the interest of the task at hand, but in violation of contracting rules, is rule-breaking and insubordination but not corruption.
"Defeating an RFP", then, is frequently (behind closed doors) agreed to be in the best interest of good government. A chosen expert—despite the bad optics—can do better (usually much better) than the RFP process.
I'm not sure "corruption" is the right word for this, since the goal here isn't to line anyone's pockets, but actually to choose the consequentially optimal, highest-ROI option. It's not quite "two wrongs making a right"... maybe "three lefts making a right?"