While it does matter how much training one has to receive in order to do a job, the laws of supply and demand still prevail. If there are many people available and willing to do a job, the job won't pay well. If the skills required to do a job are in short supply, the market will pay a premium for those skills.
AFAIK, the two umpire schools are filled to capacity each year. There is no shortage of umpires. Umpires are all too willing to make sacrifices for the a shot at "the show".
Until the quality of available umpires is poor enough to warrant a monetary incentive to ensure higher quality, the wages aren't likely to change significantly.