The problems with "just play with experienced people"? Plenty:
1. You may not know anyone who's experienced. That was certainly the case for me when I first started learning about D&D. My initial exposure to the rules was sources of questionable legitimacy for 2e, just before 3e came out. I didn't know anyone who played D&D at the time.
2. Even if you do know people who play, you may not be close enough friends with them for it to make sense to invite them to a game or to ask to join theirs. Social activities are never that simple, much to my chagrin.
3. You may not have enough close friends to get a gaming group together. In high school, I had maybe three close friends, most of whom attended other schools and lived far enough away that visiting them was a sleepover-level commitment, not a "play for a few hours" deal.
4. Even if you know an experienced group, have the right kind and strength of relationship to join a game, your schedules work out, and all the other stuff lines up....your gaming styles could be RADICALLY different. People talk up a great deal how the game should support a variety of styles, but a neophyte does not know what style they have yet, and forcing yourself to use a style that doesn't actually fit you (because you don't know any better and the person teaching you will teach you what works for them, not what works in general) is a super duper great way to never ever want to DM ever again. A similar issue applies to something like online advice, where you have to already know how to DM in order to distinguish advice useful to you vs advice useful to others vs advice useful in general.
So...yeah. There are plenty of reasons why "just learn from a pro, 4head" is either impossible, impractical, or counter-productive.
NONE of this should be taken as saying it is BAD to learn that way. I'd even hazard to say that most DMs have learned so. Instead, I am going a step further than some other posters here. To paraphrase Dr. Howard Moskowitz: "There is no perfect method, only perfect methods." By which I mean, there is no one best method. ALL of these methods should be valid, and developed as much as possible, so that people can learn in the way best suited to them, not in some top-down enforced "you WILL learn by apprenticeship(/instruction/youtube video/random reddit advice)" kind of thing.
And, as I have said in other places, I maintain that it is a bad idea to require the brand-new players to jump through extra hoops and pay extra money simply because they are new. That's a really, really good way to turn people off and convince them it's not worth their time.