Do you find it easy to hum and make up a melody? Why is that---I think it is because we've probably been singing much of our lives and humming every so often, and the voice already has a ton of options of where to go, namely any pitch you want, whenever you want. To improvise is to "hum" at the piano, but you can have many more than just one voice singing at once--so the ways in which these voices, phrases, rhythms, harmonies, combine are much larger, so it takes more work to develop the options of where to go. But I think it is a similar process.