Because congress forbids it. NASA has little control over its own budget. It probably would've cost less and worked out better if they could have manufactured a few dozen identical Spirit/Opportunity rovers with a few changes to the instrument packages and drop them all in different spots around the Martian globe. But congress authorized only two. Subsequent rovers get carved up in committee. Congressman A will vote for it, but only if a favored software company in their district gets to write the control software, Congresswoman B will vote for it, but only if the solar panels are manufactured in her district, etc.
The end result is NASA only winds up being an efficient means for the distribution of pork to various congresscritter's preferred contractors, and can seldom reuse designs.