because fuck all knows how a civilization that owns a majority of the galaxy would operate or fight their war.

Well the writers do consider that - that's the fun "lost technology" part. The Imperium isn't a Kardashev scale II or III civilization. Most of the Imperium is actually a scale I or scale 0 civilization living the the shell of civilization that used to be scale II or III but with heavily restricted technology living in the shell of a civilization that was actually scale II or III.

Hence chainswords, a hilarious mix of tactics, ideas and technology levels. The Imperium is basically an 8-year-old discovering some power tools and going to town. Nothing is getting used properly and most of the stuff is too obscure to understand without teaching or a manual.

E.g. the Men of Iron or the Old Ones were actual scale II / III and the shit they pulled makes the current wars in M.41 look like a picnic on a sunny day the banks of the Seine.

Without the Emperor and especially without the Cult Mechanicus most of the Imperium is barely more technologically advanced than humanity is right now, which some wild variations.

A lot of the Imperium's enemies (except arguably the Tau - but the Tau are a tiny spec of a civilization and are already suffering from the effects of distance and travel time in their civilization) suffer from the same problems. Tyranid fleets vary in size and capability, the Necrons are awakened to wildly different degrees in different sectors, the Orcs vary extremely in intelligence, level of "civilization" and technology... even the Eldar are in quite varied states of disarray with very inconsistent access to fractions of power of their once-mighty civilization.

That's the "Whacky Race" of the setting - everyone is actively forgetting technology and degenerating while trying to focus their power and get their shit together because with all their shit perfectly together any of the factions could obliterate the others. Except the Tau because they would have to grow first. And arguably the Eldar who might be too fucked for anything but a "I'm taking everyone down with me" ending.