Sometimes I get confused questions along the lines of "How long does a PhD take? Why does it take so long?" So, let's go through a PhD chronologically...
(I will talk about a physics PhD because that's what I know best, but a lot of this will be broadly applicable)

First, getting a PhD isn't really *school* like other degrees. Yes, you are learning all you can about a certain subject. But after 4 years of college and 1-2 years of class in grad school, you've probably learned all you can in a classroom.

The point is, a PhD isn't about learning everything already known about a subject (that's more like a Master's degree, which you would be able to earn by this point). It's about making an *original contribution to human knowledge*.

You have to go *discover* the thing that later people will learn about in class! That's really hard! Typically it entails 1-2 years of class to learn the background, then 3-6 years of working in a research group to discover something new.

This research time includes hyper-specialized study of current knowledge (more in-depth than a class would give you)--this typically takes 1-2 years. In parallel, you learn the techniques needed for research in your subfield, some of which may be specific to the group you work in

This also takes years! Why? Well, you're not just learning techniques; you're learning when to apply different ones, how to troubleshoot when they don't work, and constantly trying to improve them and make new ones.

All the while you're conducting original experiments or theoretical studies or numerical simulations or...etc. You have to do something new! Then publish, maybe repeat once or twice depending on the subfield, and write it all up in your dissertation at the end.

"Oh man, that sounds so exciting, expanding the frontiers of human knowledge!" Well, yes, it can be. But... 90% or more of your time will be spent on things like maintaining equipment, finding bugs in code, troubleshooting weird data because you designed the experiment wrong...

It's incredibly frustrating and incredibly rewarding, it pays like shit but you do get paid, it can be amazing and it can be downright traumatic. I always tell people it's the greatest career choice ever, as long as it's the only thing you could ever see yourself doing.

So, if you're pathologically obsessed with thinking about physics (I am), grad school is the best! Otherwise, there are lots of ways to use your brain, help people, get paid, and have a reasonable quality of life, all of which are easier than grad school.