The substitution might work better, you're right, but that's also effectively admitting that most jobs aren't creative. There is a qualitative difference between working close to the limit of one's abilities in a flow state, and having one's own ideas, following through the implications, and executing it using one's own wits.
That's creative, and in my experience a lot more gratifying than just being engrossed in work.
In any case I think it's a mistake to become too transfixed by what features of work are most conducive to happiness. Yes, happiness is important. Perhaps it's the single most important criteria for choosing a line of work. But there are other, larger objectives - e.g., helping humankind, producing great things, being authentic to oneself, and leading a good life.
