Because they also want to have a good Olympics. So they spend a lot of time evaluating whether or not the Olympics will be successful in a particular location. The bribes go to evaluators who say that it will. And then the location will build a bunch of venues for the Olympics.
Under your system, the location will purchase the Olympics and then be broke. They won't be able to afford all the venues required to actually hold the events. Then the Olympics will have to build them. So the locations that pay the most may tend to have the fewest venues. Then the Olympics builds them a whole set of new ones. Such locations benefit more than locations that already have venues. In the end, the Olympics might make less money (or even run at a loss).
Better might be to let a for-profit organization run the Olympics. The for-profit organization bids for the Olympic rights and then picks the location. Because it is for-profit, it can pay evaluators bonuses for a successful Olympics. So they won't want to take illegal bribes that may lose them their legal bonuses. They may even be independently wealthy. For example, Mitt Romney was involved when the Olympics were in Utah. Wealthy evaluators are harder to bribe.
The evaluation is a critical step. They can't skip it in the current system. They would have to redo the system to align ownership and responsibility such that someone else does the evaluation. My for-profit suggestion is one possibility, although it may have its own pitfalls. The problem with the current system is that evaluators aren't terribly well paid and have no real reason to care if the Olympics fail. This leaves them vulnerable to corruption by monied interests.
Another way of looking at it is that your system picks the location that is best served by having the Olympics there. We actually want to pick the location that produces the best Olympics, which might be different.