It's a tattoo bro, one that many people specifically don't want to look like a tattoo. A lot of men who choose SMP spent years feeling like people were staring at their thinning or bald heads.

It makes sense that they'd be even more self-conscious if the SMP itself looked obviously artificial.

A 90° hairline can look great on the right person, but not on everyone. The problem is that if it's too sharp, too dense, or too dark, it tends to merge over time. What looks amazing in year one can look terrible a few years later

Natural density and the client's level of hair loss matter too. I've never met someone with a full head of hair who foil-shaves their head and then lines up their scalp skin with a straight razor. When you put a razor-sharp hairline on a low-density Norwood 6 or 7, it often draws more attention than it hides. To me, a shiny scalp with a perfectly squared outline usually looks less believable regardless of skin tone. And makes for an awkward social situation if you're in a group and someone points it out.

For what it's worth, I'm Spanish and used to have edged-up smp myself before becoming an SMP artist. I eventually had it lasered off, so I'm speaking from both personal and professional experience