There's nothing much to really understand.
Graduate high school. Go get your trucking License. Go do garbage collecting. If you wanted to, you could always move up the chain. Truckers in large cities make way more then 40k. Some 50, 60k or more. In some rare cases you have truckers making 100k
The $100,000 job: Garbage workers
So lets see, you are 18, debt free, driving around a truck, have benefits, just making cold hard cash. What you do with that cash is up to them. Invest it? Buy a house. Who knows. All I know is that they are debt free and a decade ahead of you before you start earning one dime.
4 years of expensive schooling + 4 more years of expensive graduate schooling + buying a 500k-1 mil dollar practice...and the cash flow comes in your middle 30. If everything goes ideally well and you graduate 26-28, buy a practice and on the right track by 30....you could probably be debt free by 40's. This is ideal. Some do it much quicker and some don't get debt free until their 50's.
There are much easier routes in life if you want to talk about money. Dentistry is a good gig, but you are missing the point that you are paying a huge price for time effort and opportunity cost by investing into dentistry.
If you wanted to make a better argument you could argue computer science versus dentistry. My brother in law is making 250k at 24 years of age working for one of the big tech firms with stock options that matures in 4 years that would push his net worth to 1 mil at 26. Oh and he graduated with 50k of debt.
But that's comparing apples to oranges which makes no sense...but don't look down on those garbage truck collectors, they are doing a great job, and some are making some big dough. I love dentistry and I would def do it again even with the debt...the only thing I wish I tried was computer science. Oh well maybe next life time.