I'm so glad you asked that question, it gives me an opportunity to sprout.
From my perspective, there are 2 reasons.
1. Poor economics. Cost savings on welfare aside, raising everybody's income by say $1000/month does nothing more than inflate an individual's money supply. When every individual's money supply in society is inflated then naturally, prices also rise. Furthermore, wealth is enhanced when we seek to exchange value with others. In order to do so we must create something of value that can be exchanged in the first place. Being paid to create something that has no value to anyone else ie a living wage is a waste of scarce resources.
2. More insidiously, it increases our reliance on the State. Let's look at an analogy. We all love our kids and want the best for them and we would all love to gift them everything their heart's desire. But we know that is not in their best interest so we encourage independence. Being reliant on a benefactor breeds slothfulness and other vices that would certainly conflict with your protestant work ethic.