The existence of taxi drivers doesn’t discourage folks from learning to drive. I’m not sure why a DM being willing to run your game for a small charge should discourage folks from learning to DM. If anything it should be an incentive.
Excellent analogy. The existence of restaurants doesn't mean everyone dines out, but you do need to actually
learn to cook, and doing more than "it's basic and filling" requires effort, practice, and learning--good cooking is hard, but passable cooking is pretty easy. The existence of laundromats and dry cleaning services doesn't mean everyone takes all of their clothes there--but doing dry cleaning at home is, to put it simply,
pretty difficult and requires at least a little starting knowledge and some kit.
These analogies are all imperfect, as analogies always are, but they capture the core, the gist. GMing is a skill. That skill takes time to learn, and is difficult to do
well. It's very easy to just throw yourself at it...and the results may be decent, impressive, or disappointing. Getting help to become good-to-great is very useful--but you absolutely 100%
do not need to pay money for such things. There are scads of free resources, many of which are very good. (Again, I recommend the GMing rules for Dungeon World as a starting point--they won't apply perfectly to every game ever, but they are
very good as a starting point for D&D GMing.)
Further, some games are easier to GM than others. Game design matters, but it isn't perfectly determinative. Just like how genetics isn't perfectly determinative, but absolutely can influence and affect things. My family has a predisposition to colon cancer--but relatives of mine lived long, full lives without ever developing that kind of cancer. System design can help or hinder.
(And on the subject of system design--early-edition D&D was not at all easy to GM
if you actually used the rules. The difference is that people were so cavalier about using the rules that few people
actually did "play D&D" in the strict sense. Instead, they played "what parts of D&D I remember", which was often far simpler, easier, and relied more or less totally on "can the GM invent crap from whole cloth, on the fly, that manages to stay both consistent and worthwhile/challenging?" Which is, you guessed it, yet another skill! A very difficult one to learn at that, because the books were so goddamn opaque and poorly-organized. It's frankly a miracle
anyone learned how to GM given the books available at the time.)