Slaves have no freedom and cannot choose what they do, where they go, or where they live.
Taking the UK as an example, almost no one is a slave. (I would say "no one", but in the whole nation, there is probably someone being illegally kept somewhere, just like in the US, where we find from time to time some monster who has locked an unfortunate into the basement for years.)
Voluntary anything (employment included) and slavery are two completely different things.
Why so sour on today's world?
It's not perfect, but it is a lot better than how the typical person lived in the past (say, 1800's or earlier -- or today in some third-world countries). Even poor people in western countries have what those past people would classify as great luxuries. In the US, for example, the typical poor person has: enough food to eat so that obesity is the major health problem; a television; a car; an abode; heating and air conditioning; clothes; a microwave; a conventional oven; an indoor bathroom; spices; running water; electricity; lighting; carpeting; furniture; books; a cell phone; a refrigerator; a freezer; and a console game. We need to keep working to make things better, of course, to create a vibrant, growing economy with enough opportunity for all who have a desire to go for it; but the life of humans has been on a major upswing in modern times.