Mostly, it's a nuance that has legal and tax implications, but the original reason is that a tip is an optional (not required) direct gift from the person receiving service to the person doing service. It's a thank-you gift.

This is heavily diluted when you expect tips for anything else, and it makes it very obvious it's just a hidden fee that may or may not even be going to the workers. Wage theft is the largest type of theft in the US, and a non-insignificant amount comes from tip pooling schemes where owners and managers take a dip.

I tip my barber because they took extra time and attention to ensure I had the perfect hair cut and beard shaping to look good in my workplace or on a date, or whatever. I am not generically tipping for having done a business transaction, the person who took my appointment over the phone is not the person who cut my hair. I am giving a thank you gift to the barber because they took time and attention for /me/ /personally/.

FWIW, I appreciated restaurants that had a menu option to buy a round of beers for the kitchen. I would do that every time it was presented to me, above and beyond a tip. To me, when I tip someone who rendered me a personal service (especially a service where I have a conversation with them), it's me saying "I'd buy you a beer and say thank you, but I can't do that right now and you have other customers, so here's some cash as a thank you.".

Diluting that meaning eliminates the reason I'm willing to tip in the first place.