OK, herlock. Let's just agree it's out of friendship and mutual respect. Possibly, it was also convenient for both or timely in some way. It happened. They're touring together again and maybe some general music fans will regard it as an opportunity to see both together, perhaps for less than seeing the two separately.
Me, I'm very happy to see them both on the same bill and to see them both more than once on the same tour. For many fans of one or the other, however, such as yourself, this is a less happy combination. Fans on both sides of this apparent fence have been less than respectful,I suspect, which is a shame.
There is a general problem with opening acts if you're a fan of the main act and not the opening act and particulalrly if you are attending several shows on a tour. I can recall seeing Dylan at the Hammersmith Odeon for some, I think it was, six consecutive nights with Roddy Frame (of Aztec Camera) as the opening act. Now, there was nothing wrong with Roddy Frame but I had no real link with him or his music. I guess I saw him only once or twice that week - which probably was disrespectful.
On the other hand, I have also gone to shows principally for the support act.
And I certainly agree that some of the best musical experiences that I've had have been at live shows. It's not just the music, which may or may not be the best in absolute terms, but the atmosphere, the ambience and the company. At one point during a Thelonious Momk concert in London, it was just him and me - there was nobody else in the room. And the Tubby Hayes Big Band in the back room of the Bulls Head in Barnes was just physically stunning. Dylan in four shows over two nights at the relatively small Supper Club in New York - a wonderful experience. I could go on but won't.
As for why it's not just a PRIVATEERING tour by Mark, you'll have to ask him.