Why are there so many examples in the Star War's Imperial fleet where no shield generator is in use?

Why are the Star Wars' Tie Fighters built to have no shield generators and are generally weaker than all ships I have seen engaging them? Actually, why did the Star Destroyers have no shielding. Was there a shield generator shortage?

Sorry I forgot the Imperial Star Destroyers do have shields. Thanks for reminding me in the answers. I was confused when I viewed the following image.