>> "why aren't there more entrepreneurs offering a better value service? "
The problem is that as a customer how do you tell the difference between an honest entrepreneur and a fly-by-night operator? Comments on the TA forums aren't, unfortunately, always a reliable guide. There is one company offering port parking that has been subject to vitriolic attacks over their Southampton airport parking service and even large companies have problems at times.
The hotel with parking often seems to be the most cost effective option and I would be less concerned with handing keys over to a hotel than to some you can only contact by mobile phone!
Avoiding the parking problem by arriving in Southampton by train or coach is also worth considering.
The roads around the ports are not the most pedestrian friendly, especially with any amount of luggage. You may be able to see your cruise ship on the other side of the fence but actually getting to the terminal to check in might be a bit of a struggle. (There are actually several cruise terminals in the Southampton docks)