Because, for them, it’s not financially worth it. Given that they’d have to spend so much on research & development, production and manufacturing, marketing, support etc. and when you take into consideration how many would actually BUY these machines, it’s not worth the cost.
Basically, if cost of investment doesn’t equal (way) less than cost of return, it’s not worth it.
They could actually save money as all they need do is create an identical card but with no output ports on it. The design and manufacture is the same except for the back plate that has no outputs on it. Saves on HDMI, DP and USB 3 interfaces. Win/win.