If you want to raise money you need to sell some stock. So the company creates new stock and sells that -- increasing the total number of shares. You don't get any of that new stock yourself, so your total ownership percentage goes down.
Of course in real life things are much more complicated. And, if I was a cynic, I'd say they are purposely complicated in an effort to screw people over.
The founders and angel shares do not delute and your percentage goes down each round.
If your lucky they are worth the same.
[1] https://techcrunch.com/2017/09/13/how-pro-rata-works-in-vent...
they're purposely complicated, and recruiters and VCs will take advantage of people who don't know how to do the math or reason about the amount of value extraction they're capable of, telling folks "oh this stock could be worth 100x of what is now"
If the company fails, all that means nothing. If the company goes big, it won't matter. But in the most likely exit case where the company sells for less than everyone was hoping, now you get into the interesting territory where the investors might make their 1.5x return before you see a cent.
It's not that founders don't understand this. It's that founders end up taking these conditions when they're low on negotiation leverage and choose between that and running out of funding, so it's an understandable decision. However, they also avoid talking about it unless forced to because it changes how you will value your stock, which changes how attractive your offer or current compensation is. As a rule I ask about it when I talk to startups about job offers.