Because some people are really really big on personalization, aesthetics and customization. Some people spend hours tweaking how their characters in games look like. Some people are willing to pay just so they can customize character details.
And while I almost never use emoji, some people use them a lot.
That in no way answers why options of skin tone on a linear scale between latte and chestnut were chosen as the only particular option, which do not even cover all common human skin tones.
As I said, if skin tones should be customization, then simply allow for an a.r.g.b value of some sorts.
The skin tones were based on the Fitzpatrick scale, a recognized standard for determining skin color.
ARGB Skin tones are technically difficult to implement in unicode and would explode the amount of work needed by emoji font designers. It's always a trade-off between enabling expression and keeping it practical.
That's really interesting! It's nice to know that the tones were chosen from an established dermatological scale that was created to help people keep their skin healthy. That's about as wholesome and rigorous as you could hope for, I think.
And while I almost never use emoji, some people use them a lot.