Lets look at it this way. Grandma gets a whole new OS pushed to the phone that she's taken a year to learn how to use. Now something isn't working correctly. She's going to call the support line. Since it's a bug, they can't help her over the phone, and can only say "wait for the fix to be pushed to you in a couple days". Grandma gets frustrated about it. "Really quickly" isn't quick enough when you don't know enough about your phone to work around the issue. Grandma isn't going to think "let me google this error and see what I can do about it myself".
Fix comes, something else stops working. Repeat process. It costs lots of $$ to field those support calls. More money spent on support is less money spent on the network upgrades, less features allowed (data caps!), or higher bills.
There is a LOT of Quality control that goes into an official carrier release of the software. It may not seem it, but a vast majority of users just know they have an Android or iPhone. Some don't even know that their Android isn't an "iPhone". Much the same way that every mp3 player is known as an iPod.
THAT SAID:
Our developers are awesome that they can get a nearly perfect OS out to these devices. Their target users know enough to work around, or understand that things may not all work right off the top.