Because it leaves you with no other education, but aviation. When the aviation sector takes its inevitable dump, you are left with nothing to fall back on, UNLESS you have some skill in another area. Even after 9/11 and airlines just started to comeback, they were hit with the SARS scare in Asia. We had a 747 pilot in our A&P class during that time. Also, what if you suddenly have a health issue and can no longer fly; what's your plan then? I know of 2 guys in their 20's who lost their medical.