In just over 50 years Earth military aircraft advanced from those that hardly could stay in the air at a mere 70 MPH, to a vessel that set a record of 2,193.2 MPH. Not only did simple speed increase by orders of magnitude, but various other aspects of aviation also surged ahead at exponential rates, and warfare technology elsewhere blossomed at rates possibly not as impressive as Aircraft's early days, but still extremely worthy of note. The growth of warfare has been so fast over the last hundred years or so that a difference of by a couple of decades would mean absolute suicide for one side for traditional warfare in almost all cases.
Despite this apparent fact in our real world, many soft-science fiction worlds, such as Star Wars, Buck Rogers, Warhammer 40k, or even Star Trek at times don't seem to suffer from this effect. Sure, when centuries separate technology sometimes that's an issue, but even then typically the warfare as the viewer sees sees hasn't advanced meaningfully; newer ships still buzz around more or less as fast, and generations of personal armament don't seem particularly more or less effective. Star Wars with the lack-of-apparent-change Old Republic to the original Star Wars trilogy being a particularly bad example of this, as thousands of years separate the two, yet warfare seems by-and-large just different flavors of one another.
While sometimes authors of such settings include mention that warfare has advanced over the years, but they typically have it as a footnote without real proof. Instead of hand waving such advancements in place, what if we desire this lack of Earth-like advancement in warfare?
In a setting of a galaxy's worth of different cultures and societies and where war isn't a scarcity or oddity, how could one explain a lack of scientific advancement over the years as it applies to warfare?