Starting with a manned aircraft is a terrible way to build an unmanned one. A good fraction of the weight is there to provide life support and safety for the pilot. The canopy is a huge compromise between aerodynamics and visibility. And for manned aircraft, it's worth spending 10x more to get a slightly lower failure rate, because pilots are so valuable. For unmanned aircraft, the reliability calculations are all different.
But it gives access to thousands of hours of training data for that particular aircraft across a range of different situations, and it allows you to test that ML model in real situations.
Moving to something lighter or more aerodynamic becomes a lot easier after that. If you look at the Boeing Airpower/Loyal Wingman UAV, it looks like its based on this concept quite closely.
That doesn't seem to be the route the XQ-58 is going, though. [0] It's an attritable stealthy missile/bomb truck - and at a projected unit cost of $2-3 million, they'd be cheaper than some US air-to-air missiles.
