Why are there so many vehicle variants?

I was thinking about all of the TIE variants and trying to figure out how comes there are so many variants of any kind of fighter craft when in the real world we have far less variety in our own air forces.  This line of thought got me on to thinking that this can be used for more than fighters but ground vehicles, shuttles, small capitol ships ect.

To start out with we need to look at how different the manufacturing base of the galaxy is very different from our own.  In our own world we have to mine materials, ship them from around the world, manufacture parts, ship those parts to a central plant and then manufacture the product be it a car, computer, phone, or tank.  With our  production plants we need trained workers or robots that are specially designed to make whatever specific product they need to make.  Now in the Star Wars universe we have a galaxy with unlimited free labor and resources a plenty.  Droids can run automated manufacturing plants as well as droids being able to mine any number of uninhabitable planets and asteroids to gather raw materials.  The only materials that would be expensive would be things like tibanna gas, kyber crystals, or coaxium where you wouldn't be able to find these materials in any given star system.  

So the Empire can literally set up a manufacturing base in any uninhabited system and drone mining ships can go out and collect resources and bring back materials for an automated factory to make tanks, blasters, armour, fighters, ect.  How then can a corporation thrive and exist when anyone can get an automated factory, down load some schematics and hit the on button and start churning out war machines.  Well we just answered the question right there.  Many corporations don't exist in the way we think of.  Instead of being a traditional company like where they own the factory and run it, staff it, make decisions as to what to build, the Star Wars universe corporations would instead build factories and supply the owners of the factories with administrative staff and of course the schematics to whatever needs to be built.  

Sienars product catalog

Ultimately this is what corporations like Sienar Fleet Systems actually does.  They do R&D then sell the designs to the Empire.  The Empire will also buy a Seinar automated factory, park the factory into a sector, upload the schematics and start filling orders for local sector commands.  I would guess that Seinar would also have a royalties agreement with the Empire and have a small fee paid in royalties for every unit produced. 

This would explain how the Empire is able to have a near unlimited supply of light craft.  The Empire would never have to worry about running out of war machines minus the large capitol ships which would of course be far more involved in the building process. The real problem for the Empire in filling out its military rosters would therefore not be a material issue but an issue with just having enough trained personnel.  But thats a topic for another time.