Because until and unless PC makers start favoring USB-C ports then there is little incentive for peripheral makers to follow suit. HP makes the Spectre that has only USB-C and Thunderbolt 3 ports.
It isn't possible to offer Thunderbolt 3 (or any Thunderbolt) or USB 3.1 Gen 2 over USB-A ports, with or without adapters. But it is easy to offer HDMI, DisplayPort, Thunderbolt 2, USB 3.1 Gen 2, and earlier standards over USB-C. And there are native cables that offer USB-C on one side and HDMI, DisplayPort, Lightning, etc. on the other. It's an industry standard, albeit a new one that hasn't yet achieved widescale adoption.
I can't believe people like you would think that a company that dropped EVERY legacy port in 1998 in favor of USB wouldn't do the same 18 years later when a newer and better industry standard came along. The writing has been on the wall since April 2015 when they released the MacBook with only a single USB-C port (not even Thunderbolt 3). They sang the praises of a single port for I/O and charging. And now they offer 4 of the ultimate port.