To be certified as a training device it has to be accredited to act similar to the real thing. That accreditation costs money.
That's not to say it couldn't be done cheaper, but until someone does $15 million is what the market will bear. The other option is gassing up a real jet for training.
Also, who accredits the simulator? The company that built it?
But $15 million is still expensive. A major selling point (possibly the biggest selling point) of the MAX jets was that Boeing convinced the FAA that they were similar to old 737s and did not need a new type certificate. That meant for the many pilots already certified to fly the 737 they would not need to do any flight training to transition. All those currently certified 737 pilots could fly the new jets just by reading a manual on an iPad for an hour. That is a huge savings in training cost even compared to powering up a simulator. The simulators still exist because new pilots still need to get certified need to log flight time- the iPad app is not good enough for them.
Regarding simulator accreditation, I believe Boeing does that themselves. They hand the FAA a stack of documents claiming that they tested the simulator with the same flight profile as a real jet and it handled the same as the real thing. The FAA basically says, "as long as you're not lying about running the test then the simulator is acceptable for training." This is how the FAA handles most certification- they do not do their own testing, they just accept manufacturer test results and documentation. As long as the documentation claims to meet standards then it's all gravy. In general this system works, because if someone crashes your plane and a whistleblower says that you did not actually perform the correct test procedures then you are in hot water.