For the patent holder who’s an independent inventor, trying to interest someone in investing in their idea (or buying it from them), yes, they’d have a vested interest in making the idea of their invention readable. But, I’m not sure that they’d be using the patent, itself, to do that. If they’re trying to sell the idea, they’re going to work up other materials, demonstrations, etc. for sales, rather than just trying to let the patent sit out there and passively attract attention. (Or, at least, they should be, if they actually hope to have any chance of selling their idea.) But, even then, I would bet that those sorts of things are things that a smart investor is going to want to keep tightly under his own control, rather than just having it widely published, in order to discourage others from attempting to do something similar.
Besides, I wouldn’t be surprised to learn that independent inventors make up a tiny percentage of the total patents issued. For everyone else, the “fear and doubt” which Reply mentions seem to be likely reasons to keep things obscure.