* Don't buy bread, because someone who knows much more about bread (the baker) would much rather sell you the bread than eat it themselves.
* Don't buy a car, because someone who's much more familiar with the car's design (the manufacturer) would much rather sell you the car than drive it themselves.
* Don't buy the book, because someone who knows its content (the author) would much rather sell you the book than put it into their own shelf.
Specialization exists in all but the most primitive of economies, so while not every loan is a good idea (obviously), many loans benefit both sides. A bank doesn't make a profit from money just sitting in accounts, that's why they loan it out to someone who can use it to make profit – like a company that wants to buy a new machine to work more efficiently. The bank could buy such a machine as well, but it wouldn't be able to use it in a profitable way.
A bank won't use a machine to manufacture something, and perhaps you have the skills needed to do so.
But deceiving yourself that you can "borrow cheap, invest it in the stock market, and always come out ahead" - if it was always or even for the most part, why would anyone lend money and not do it themselves?
Because you're missing risk.
Because there are regulations that limit how banks can invest deposits.
This text is a pretty good example of the “smooth seas don’t make skillful sailors”.
My credit card limits were reduced like 90% during the 2008 crash.
Even big corporations that have contractually committed credit facilities have had banks shut those down during times of crisis (basically saying “sue us” to the borrower) so no matter how solid the ability to borrow may appear, that kind of thing has a tendency to evaporate in a moment, just when it’s needed most.
I have credit cards with a ridiculous aggregate credit limit, but in another 2008 scenario, anyone want to take bets on how long those credit limits stay high while unemployment is on its way to 14%? I would prefer not to.
And regardless of earning potential or useful skills, the owners and managers of for-profit enterprises are going to be belt-tightening at the same time (out of necessity or caution or simply by never letting a crisis go to waste) and their appetite for bringing on a new entry level employee will be limited - just at the time the rest of the system is grinding to a halt.
Someone in their 20s without significant financial or familial obligations is generally going to be well situated to make it through one of those crunches, but it doesn’t take a whole lot to bring the edifice crashing down.