As you say, neither Dylan nor Knofler has to tour for financial reasons (and, these days, there's more money in touring than selling albums). Knopfler usually tours in support of his latest album but I'm sure he doesn't do it just for the ticket sales or the album sales touring generates. It is more likely that, having produced some new songs, he is understandably anxious to get them better known and more widely appreciated. In that sense, I suppose one could say that his tours are for artistic reasons. But, yes, he does it because he WANTS to and, presumably, to keep that connection with his audience and his fans.
As you say, Dylan does it because he wants to, as well but with a difference. Dylan just tours and tours and tours. There doesn't have to be a specific reason. He does introduce new songs on stage once an album has been released (the delay is more because he doesn't want his new songs to be bootlegged in advance) but a more sporadic way.
In the very first year of his recording career, he said that he hoped to be like the old bluesmen of his day, performing publicly into his dotage. Another factor here is that Dylan has never had what I might term a "real" job. Apart from a couple of very short-term temporary jobs when young, he has only ever been a recording and performing artist. It's all he has ever done.
In the sleeve notes of his first album, he said he just wanted to keep singing and writing songs and on his second album, I think, he said that he didn't carry himself the way some of the old bluesmen did but hoped to do that some day. And that is what he has done. I mean, it can't be too bad to have written "Make You Feel My Love and to have had it taken into the charts by others as you're approaching 70.