>>> why would you?
Because
(a) they can slow a computer to a crawl while they're being downloaded and installed ; and
(b) if someone only has a fairly slow broadband connection, they can make services such as Now TV, Netflix, etc unusable while the downloads are taking place.
The first thing I always do with any Windows computer is to turn off automatic updates. I can then choose what (and, more importantly, when) to download them later. However one of the many things which put me off switching to Windows 10 (and why I'm still using Windows 7) is that users CAN'T turn off automatic updates. Sorry, Diddlydo!