Perhaps you just found an old dusty upright piano next to a dumpster, and you moved it into your apartment. Pehaps you pay a "very expensive" teacher $50 and make a "large time commitment" of 30 minutes for a single lesson. (Btw-- what's the going rate for even moving an upright these days? I'd bet it's more than $50)

The teacher asks you where you'll be practicing, and on what type of instrument. Suppose it's the old upright. As a professional responsibility, that teacher is going to find out exactly what shape that piano is in, if it's in tune, if it has a broken soundboard, etc. Based on what the teacher finds out, they will give you valuable feedback on whether it's even worth it for you to devote learning on that instrument (perhaps it's woefully out of tune[1], which you wouldn't know if you're just a beginner). As well as whether it's worth paying a piano tuner to have a look at the instrument. Tuning costs more than $50, so on that detail alone you may have gotten your money's worth.

Now imagine that instead of an old dusty upright, you have access to a grand piano practice room at a college with a well-funded music dept. You again pay your astronomical fee of $50 for the long duration of 30 full minutes. But the teacher spends the last 10 minutes of your lesson talking about dusty old broken uprights, and all the problems you'd run into if you happened to be playing on that instead of accessing the college's practice rooms.

As a beginner, that last ten minutes would clearly be a waste of your time, right?

Now extend that waste of time over, say, 1/3 of the total time you attempt to learn the piano.

That is pretty much the problem you run into with online/self-study. Either the guide gives you so much material that's irrelevant to your context that it slows your pace of learning, or-- probably worse-- it leaves out crucial details that you need to know in order to progress efficiently.

Finding a human mentor through a web of trust avoids those problems.

In fact, it gets worse over time for the online/self-study because you're entirely dependent upon your own opinion of your progress, which-- as a beginner-- is guaranteed to be flawed. This is an easy route to burn out. Perhaps you feel your progress stunted, whereas a professional could tell you that you've simply underestimated the increase of difficulty of a particular skill. Not to mention setting reasonable expectations for progress in the first place, and a hundred other considerations that a mentor can immediately discover and provide feedback for.

[1] More exponential explosion-- there are various ways in which a piano may be out of tune. A teacher can tell you whether that happens to be imperfect yet usable, or whether it renders it completely worthless as a practice instrument.

Edit: clarification