Because these new ports can do everything. Why is it "pro" to be locked in to specific use-case ports? I need VGA. Actually, there are times when I need two VGA ports. Does everyone need to suffer because my job has outdated equipment? Just a simple cable gets the job done. And that same port can be used later as SD card reader, Ethernet, DVI, HDMI, DisplayPort, 8-port USB-A hub, and many many more options.
This is exactly what a pro system should be doing. Adapting to the user. I RARELY use HDMI, why do I need a dedicated HDMI port on my system? I rarely use an SD card, why should there always be a dedicated SD card reader on my laptop? These USB-C / TB3 ports are the entire definition of what a pro system should be. User A does needs port X, and User B needs port Y. Both can be achieved with these standardized ports. The laptop does not need dozens of ports on it.
The answer to your question—and the demonstrated flaw in the your logic—rests with the fact that you say two contradictory things:
1) basically, paraphrasing, "I don't need a bunch of the ports that were offered"
2) "The laptop does not need dozens of ports on it."
More varied ports means more flexibility. For everyone. (Whether each person needs or benefits from that flexibility is a separate question.)
Now, you do cite a use case that "more varied ports" doesn't inherently address: namely, something like your need two VGA ports.
But the great thing about more varied ports combined with a standard like USB-C—and the very approach the Surface Book 2 takes—is that you can address that. A couple USB-C/TB-3 ports combined with ports for legacy equipment is a win-win-win situation for nearly everyone. And then if you WANT to carry an additional dongle around for the real edge cases, you can! But most users wouldn't have to. BY CONTRAST, the TB MBP forces nearly 100% of people today to use dongles.
A couple USB-C/TB-3 ports, a SD port, and a couple USB-A ports, and a MagSafe is a pretty ideal setup for 2017-2018. (And whaddya know--that's exactly what the old models had, with the exception of it having a separate HDMI port and the USB-C being TB-2). And if you want to keep your laptop around beyond 2018 (i.e., as things switch more and more to USB-C), you could carry hubs and dongles to expand your use of those two ports.
This is, of course, academic, as Apple is notorious for doubling down on its decisions. But as this thread is about the Surface Book 2, I wanted to point out the reasons why I think their decision is a super smart one.