After a couple of times turning a $400 brake job into a $100 brake job by DIYing you're gonna have a hard time swallowing the price differential on further repairs. Experience builds from there. Once you've got enough experience to have an understanding of the underlying systems there's not much worrying because you know the state of the vehicle you're driving and you know that you're unlikely to be surprised. Most people either go through this when they get their first shitbox car early in life or never.
You do that brake job once every 2 years or so. That's not a lot of time to really get into the habit. Especially since it comes with the worry that you're going to do it wrong and cause a lot of damage.
In the past couple of decades, cars have become a lot more reliable. They just don't need that kind of repair very often. It's a lot easier to let a professional do it, and save yourself the hassle of storing ramps, oil wrenches, etc. for 99% of days that you don't use them.
That changes as the car gets older, and you need to decide between another $1,200 mechanic repair, a $300 self job, or a new vehicle. If you're into doing the repairs yourself, you can save a ton of money. But that only occurs once a decade or so these days, and it's just not worth it to many people.
If you start with a $1300 beater that needs brakes to drive reasonably and a whole laundry list of "this is starting to go and might go next week or next year" items you can pick up the experience as fast as your budget allows.
When you only have hundreds in the bank paying someone to do a timing belt when the kit of parts is $120 just isn't an option. You either do it yourself or you cross your fingers.
