Did you know women were shaving with safety razors in the 1950s? That is, until manufacturers learned they could make almost any product quicker and cheaper with plastic. These were way more lucrative for manufacturers since they had to be replaced far more often. Shorter product lifespan = more purchases = more profit. Nevermind the havoc they were reaping on the earth by sitting in landfills. With access to cheaper materials, brands and advertisers began convincing women the more blades, the better.
We get the logic — more blades = better shave, but it’s simply not true. More blades leads to irritation, specifically razor burn and ingrown hairs. Dermatologists dig into the details in this New York Times article too. TL;DR: Multi-blade razors often cut through too many layers of the skin causing razor burn. Or they cut the tip of the hair too sharply, causing it to curl back and grow inwards creating ingrown hairs, aka razor bumps.