And you don't have to install any drivers with most PC graphics cards either. They just work. But you'll never get the most out of one without the manufacturer's specific driver, as they can use the undocumented features of their hardware and firmware that they do not want to reveal to third parties.
Because many motherboards (often at less than £50) come with onboard graphics and for those that don't a £30 graphics card often suffices. They are perfectly suitable for most use (web browsing, HD video, office work, even low end gaming), but may not handle graphics intensive games at high resolutions, high frame rates and with the highest quality effects. Those sort of cards cost hundreds to even thousands of pounds each, but are not required by very most people.
Similarly, almost all PC motherboards come with on-board sound and of decent quality, but a small number of people will want to add specific, extremely high quality sound cards or ones with special features.
SteveW