Several reasons
1. There are several kinds of cables, I had to order 2. One worked on my android chipsetsand another worked with my Asus ROG with AMD. HTC vive don't know what you have. Putting a cable that only a small fraction would use is not cost-effective in a hightly competitive VR market. Think of that long sync cable you likely have and not using - you don't want to bundle all that. Also the cables have converters, its not a $3 cable, its about 3 $10 each on retail.
2. The USB-C to to display port connector standard came out on September 2014. That is 2 years before the vive came out and likely was coming out during the design of the vive. The could not anticipate if people would actually use it.
3. If I would complain it would be to Asus that 1. didn't document it well at all. 2. When I got Asus Zenbooks they came with the VGA adapters, this would be a similar cable missing. That and the camera, not sure what they were thinking with that.
But honestly, once you know this you get the cable and get on with life. I was on lockdown with the VR I could not use and no way to get cable, I feel your frustration.