What he said…It is not odd. Pretty much no quick lube place tops off coolant anymore.
There are probably 5-6 reasons I can think of.
A big one is this.
Coolant bottles used to be unpressurized "over flow" bottles. In the last 25 years coolant bottles known as "degas bottles" are the norm.
Degas bottles are integrated into the pressurized side of the cooling system, and most if not all have the "radiator cap" on them. it is dangerous to open a hot pressurized cooling system.
When ever you have a question, the answer is almost always liability/safety.
Ugh don't do that. That shit is terrible, has solids in it and will eventually coat the system with a white funky haze. Did many tests with various water surfactants back in the day. If you are going to add anything like a water wetter I would recommend Hy-per Cool Super Coolant.Top off with some redline water wetter.