I had a chat with an optometrist friend once.  She was adamant that everyone should see an eye doctor every 2 years at least, as so many major diseases can be easily diagnosed by the eyes.  I told her that the only reason I was getting a checkup after 12 years was that we were friends, and that if it was so important, why wasn't it covered by health canada.  She told me that when national healthcare was first put on the table, optometrists and dentists held a conference and decided they didn't want the extra patient load and that they wanted to be able to charge whatever they want, so they lobbied hard to be put in the same category as chiropractors, acupuncturists, and unregulated charlatans.  So short answer is greed and laziness.