Because not everyone has the time, tools, money, space, etc. to do their own wrenching. I llive in a 2-bedroom apt. and there's absolutely no way my wife would let me turn one of those bedrooms into a bike shop. I don't get why not but there it is.
Do it outside. You don't need a special room. All the tools you need for a basic tuneup would fit in small bag and would cost less than $40.
And everyone has time. That is the worst excuse in the book. If it's important, you will find time. If you watch TV at all, you have time. If you go to the gym or workout (including riding your bike), you have time.