Because I have to have this funny thing called MONEY to buy FOOD, because I get HUNGRY without it. Right now, our whole society, actually, both of the big ones - eastern and western, are predicated on the idea that you get paid only when you do something of value for someone who has money. In exchange for the work/product/service, they give you something of value, so you can now go get your food/drink/whatever you want.
Changing society so everyone just gets a certain allotted amount of money is considered a) socialism (which is evil according to many), and b) means I will sit on my ass all day long doing nothing, because why would I do anything else? The machines do all the work that matters! I could just sit here, ask my robo-butler to bring me the gourmet food my robo-chef cooked up for me, and I will turn into a fat slob who can't get off my bed/couch/floor. After all, it's not like I *need* to go anywhere - everything I *need* is provided for me by robots and free money/food/water! An alternative is c) communism, where the government assigns you a job (much like joining the military - you want to be a computer geek but they assign you to clean big guns and swab the deck) and in exchange the gov't gives you money/food/water and whatnot. Also not really something most Americans consider "good".