It's to be expected in a galactic civilization that's been around for at least a thousand generations. While there have certainly been advancements (and probably more than a few setbacks) over the millennia, there's a limit to how advanced things can get. Also consider that while conflict can certainly spur on technology, it can also impede it. With all the galaxy wide conflicts over the centuries, much would have been lost and gained in the to and fro of war.
Think of it this way, the basic design of the wheel hasn't changed much since it first appeared something like five or six thousand years ago. Sure, the materials have changed, building techniques have been refined (the vast majority having occurred in only the last couple centuries--really it changed very little for several millennia) but the basic design and function is essentially unchanged. A wheel is a wheel.
Likewise, a starship is a starship. They're only transports after all, it's not like they have to push the bleeding edge of technical proficiency.
Besides, it's entirely possible the ships are themselves thousands of years old. As static as the technology is it also appears to be very robust. There's certainly a precedent as during the Clone Wars, at least one of their training ships the Jedi were operating had been in service for at least a thousand years.
I do sometimes wonder if part of the reason some things seem not to change is because certain technologies only come from droid foundry worlds that have been churning out the same components for uncounted centuries. The principles behind the designs forgotten in the gulf of history and the original plans long since lost except to the memory banks of the foundry computers, who's format is so archaic as to be unreadable by modern systems.
There could be some things that nobody understands the underlying principles any-more, but know perfectly well how operate it.
Such is the perils of robust technology and an over-dependence on automatons.
