It’s all about angle.
Basically, you can cut or scrape yourself fairly easily with an old double-edged “safety” razor. (“safety” means you can only cut yourself 1/16" deep) Typically this is when you get the angle wrong and go too steep and just cut into your skin, or if you get the blade somehow not square to the direction you’re moving and then get the angle wrong. Safety razors also help the shaver keep the angle correct so that there’s a minimum of scraping, and a maximum of cutting.
It’s kind of like how you can scrape stickers off of things that a single-edged razor might scratch if you got the angle wrong, like a painted plastic bumper on a car.
Old-school straight razors totally rely on the shaver to regulate the angle and depth- there’s a reason they’re called “cutthroat” razors- they’re effectively a very sharp non-pointed knife.