Looks to me like the short answer to the question posed in the headline is that matter is composed at an atomic or subatomic level of energies similar to light

Looks to me like the short answer to the question posed in the headline is that matter is composed at an atomic or subatomic level of energies similar to light and so it would be quite inconvenient to try to propel the mass as a whole faster that its constituent parts can go. A similar principle may well be true for straight-line inertia, which might be caused by the same as the force that makes gyros stay on course. In general the scientific discussions seem to touch these ideas but approach theories in reverse of intuition so it's hard to know if they align.