If google made it possible to easily revert a vendor phone to "vanilla" android - i..e. getting rid of all the uninstallable junk the vendors put on, then a lot less people would want root.
Handset manufactures & service providers have both added lots of (by default uninstallable as "system apps") unwanted dross to low end android phones I have purchased in the past, and rooting was only way to free up some space & improve performance by getting rid of those (many "always on") junk apps.
It is not such a major issue of more recnt higher spec phones, but still wasted space & needless battery hassle
Plus there are the legit things that are hard to do on non root phone that should be possible
e.g. want to do proper sniffing to check for malware - hopeless on unrooted phone (jhave to do workaround such as phone on wifi only and then sniff your local wireless traffic instead using a non android device)
e.g.only way to stop some "always on" apps / services is to have root privs to be able to tweak the settings