There are a lot of reasons. Too many to set out cogently here, but I'd suggest two of the main ones are:
1) Cost - It is incredibly expensive to piece together a promotion. It's not really a case of asking two guys to show up and fight it out then asking if any networks want to cover it. You need staff, equipment, insurances, the ability to pay (lucrative) fight purses to attract talent. Then you need to market your event and convince people to attend, which doesn't happen for free. On top of that, you need a promoter's license, and you'll likely need at least a lawyer who is familiar with licensing law and capable of dealing with whichever regulatory body would be in charge.
All of this stuff is a hassle, and the fighters who would really benefit from this (the lower tier fighters) don't really have the disposable or the time to work it all out. The elite level fighters simply don't need to, as they can leave all that hassle to a promotion and take home decent bank.
2) The Jeff Jarrett effect - How the hell would a group of fighters match-make effectively? Every fighter, pretty much, thinks they are the best. Everyone would want to be fighting in the main event. Even some of the more realistic fighter-promoters like Sefo book matches that they probably shouldn't be in. It would be a mess without an effective booker, and that person would have their hamstrings cut if the guys they were trying to book, and possibly even cut, were also owners. Never let your inmates run the asylum.
I imagine other folks will have a plethora of other reasons, but those jump out to me straight off.
