Because every printer mfgr has a complex driver system that doesn't work the same as normal driver installation.
You can't simply install the driver at all. You can't choose "Have Disk" and install from INF at all. The only way to add a printer is during the "plug the printer in now" step of a massive installation program.
This is why I love AirPrint on MacOS/iOS. I have two printers that were originally bought for my Windows PCs (a $40 Epson inkjet and a $200 HP color laser), but I did make sure they supported AirPrint for my iOS devices. I had to download and install drivers for all five of our Windows PCs.
I brought a MacBook Pro home from work and it automatically found and worked with both of my printers with no drivers needed.