The analysis part of engineering is, thanks to modern computers, practically "done". However, we are still not able to do synthesis well. We also have exact mathematical models for that, but it usually amounts to NP-hard problems (think integer optimization).
Answer: because the exact mathematical theory can't give you models, just the performance of a given model.
Calculating EM equations exactly is also a GIGANTIC PAIN IN THE BNC connector.
I don't know about this specific case, but often the exact models are so complex that you can't optimize them analytically, so you resort to a global optimization best effort, where the results are not guaranteed to be any good - so you might as well try something DL-based too, to see how it would perform.
Classic traveling salesman problem cast into a large number of analytically incompatible and often hard to quantify dimensions.