ANSWER: Because it is advantageous.
Uhhh, not always. In fact, most military aircraft could have been built with far more speed than they already have, except that they sacrifice TOT (Time Over Target) due to high fuel usage (as most of our present fleet needs to use afterburner to get to Mach speeds... which the F22 doesn't). So the average engagement would take place at sub-Mach speeds (especially since any close-in engagements take advantage of a aircraft's "corner velocity," i.e., the speed where it turns at the fastest rate. Corner velocities tend to be well below Mach 1. So speed isn't everything. But loiter time is very important... so much so that the US pioneered mid-air refueling to increase it.
What's the loiter time on your civilian space-planes? How long can one fly CAP over an important tactical or strategic target? Your religious zeal for these space-planes is based on a very narrow and flawed understanding of what combat aircraft are actually tasked with. But I'm sure some of the former flyboys on FR can tell you that better than I can...