Well that's also because FS2, being a game, wanted to wow us with new ships, and get the awe factor. In the real world, we know that's not always true, as the military constantly makes different variants of a tried and true design, which Lucas was aware of at one point. Basing his ship combat on WWII tactics, I think the influence for variants also stems from that. Look at the Spitfire, there were probably over a dozen production variants and at least as many that never quite made it to production. Many parts of the Star Wars universe show similar traits for reusing the old, until you get to the new trilogy of course, where they didn't even bother using the old at all. I'm of course referring to the lack of all previously imagined Old Republic era starships, and instead how Lucas decided to create all new ones. /rant