Drivers are different. They work similar to an interpreter. So if you think in those terms then Windows XP may speak American English and W7 32-bit speaks British English and W7 64-bit speaks Italian while your printer speaks Chinese. It is up to Epson to write an interpreter for each translation.
They already made their money from you for your old printer so for them to write drivers for old hardware only happens if it is in their best interest; ie if they weigh the amount of customer satisfaction for people still maintaining that old hardware verses the costs of providing such a driver and the potential sale of new equipment. Generally this is one extra OS generation, so if you had your computer with WinXP and skipped Vista then they may not provide functionality for W7 64-bit (FYI: a Vista 32-bit driver should actually work if you install W7 32-bit).
Not only that but the driver for printing is different than the driver for scanning so it is actually two drivers needed to get your device to function. And that is just the drivers to make it function; if they offered additional software with "Bells and Whistles" then that may also have to be rewritten to work on new a new bit-size, etc.
You didn't specify whether you have W7 32-bit or 64-bit and you didn't specify the model of Epson printer so we can't really tell you whether a driver exists or not.