The employer/employee relationship is a business transaction: I pay them in labor, they pay me in cash. PTO is just me adjusting the amount of labor provided.

Imagine I walk into a spice stall and say: "I'd like to buy a pound of cinnamon, please." The seller might reply "Sure, that'll be $20 dollars". But I don't have $20. I have $10. So maybe I counter "Hmmm, best I can do is $10 dollars". The seller could then counter offer "I'll sell you half a pound for $10", and then we could both shake hands and walk off happy.

Replace cinnamon with labor, and that's a job negotiation. An employer, to satisfy me into selling my labor, has basically two levers: They can pay me more, or have me work less for the same pay. "Have me work less" is PTO.