Why didn't millennials just buy homes when they were cheaper, instead of waiting until prices went through the roof to even consider buying?

same as you , you wont bey a house now , but in 20 years time who knows what they will be !!!!

Excuses & fear mainly.  

"It's a bubble."

"Prices just went up.  I'm sure they'll come back down."

"I want to find a wife first."

"My parents need me at home."

"OMG it's too expensive" (without looking to see if it really is or not.)

Student debt levels, low income compared to house prices.  It is not a demand issue, it is a supply issue.  We simply do not have enough houses in the places where people want to live.  There are plenty of houses in places where no-one wants to be.

My first mortgage was at almost 9%.

A 150k mortgage at 9% has the same payment as a 300k mortgage at 3%.

House price is only ONE factor in how much it costs to buy.  Interest rates are at historic lows which is a HUGE variable.

I would much rather have people waiting longer and paying more in the end to own a home than the idiotic people that don't have the money saved, credit established, budget experience, and humility to buy what they actually need and not what they "want".  Usually, that kind of mentality requires experience (good or bad) and patience.

Because "when homes are cheaper" is during recessions and downturns when the economy is bad and people can't afford them. That's why they're cheaper. Because people can't afford to buy them. When prices "go through the roof", is when the economy is good, and people are making money so they can afford one then. It's not really that complicated. 

Student loan debt.

Some people act, but most people react.  It's just another panic buy-in, when one lemming does something all the rest have to follow.  Covid has perhaps made people a little more rabid but it's more or less a normal market phenomenon.  The market will correct itself at some point.

If millenials had bought homes 15 years, they would have lost money. Not a wise investment.

Well, my friend, not everyone is as omniscient as you. Predicting the future doesn't come easily to most.