I’ve never met a good GM who wasn’t a bad GM first, though. Myself included.
I have.
I've met several DMs in fact who "hit the ground running", and also several who, despite years, or even one case, decades, of experience, were still absolutely terrible DMs.
There's a difference between "good" and "perfect". Every DM has stuff to learn, sure, but with good advice and a fundamentally good attitude and/or the right mindset, you can be an extremely decent DM immediately. Especially if you've been playing RPGs a long time - but I can think of one person who came to DMing having only played less than a year of a single D&D campaign, and was immediately writing very good adventures and running them really well. She was very smart about DMing though and had read and thought a lot about it. The only mistakes I saw her make were in not realizing how players respond to certain stimuli, and I've seen experienced DMs make similar errors.
So I think it's just not true to suggest all DMs start bad and get good. Or that all DMs improve really even.
EDIT: To go further, I think there are a lot of transferrable skills that contribute being a good DM - those skills transferrable in both directions to some extent - i.e. learning to DM helps you with them, but if you have them before you become a DM, you'll be a better DM. For example (and these are just a few):
1) Being good at cooperating with others in a small-group situation.
2) Being fair-minded, empathic and compassionate, and not an egotist.
3) Being creative and enjoying creativity from others.
4) Being organised and prepared.
I could go on, but whilst TEENAGE BOYS (which is probably what most people are thinking of re: "bad DMs" or when they started DMing) rarely possess these skills, ADULTS, especially ones in their thirties and forties, often do possess some of these skills and others which contribute to being a good DM.