The short answer is that while parts of Data's body are relatively easy to fix and/or replicate (something we see on several occasions) his Positronic Brain was not.
The only three stable positronic brains created in Federation history were all "Soong-type" androids; Lore, Data, and Juliana Soong. The one time an effort was made to replicate Data's positronic net (TNG: "The Offspring") the android that Data
created, known as Lal suffered a massive positronic cascade failure, and ceased to function.
Data was able to access a store of parts from his father's laboratory (see pic below from TNG: Datalore) but there's no indication that any of these contained a redundant brain.
And yes, that's really Brent Spiner's butt in the picture below.