There are. One that runs over a long period of time is the Greater Ozarks Open. I dont know the amount of entries or prizes paid, but I do know that they pack 2 centers in Springfield, MO on a consistent basis for at least a few months, run piles of brackets, and have a fairly challenging house pattern (oxymoron? not always).

We don't necessarily need to START a midwest bowlers open, but expand one that is already running to become what you're envisioning.

The hard part would be to get a center to commit to a tournament of that scale before the results are proven. To get a center to cancel any other weekend events they have planned, maybe even friday night leagues so that they can guarantee squad times would be difficult, but there is no reason an already exisiting tournament couldn't grow. I already know of small groups of people that travel from parts of kansas, oklahoma, texas, and iowa to bowl this tournament yearly.