Why didn't obi-wan or Yoda recognize R2D2 or C3PO?

In the books, in cloud city, Vader saw C3PO disassembled and it reminded him of the droid he built as a kid (not knowing that was the same droid), and told the troopers to give the parts back to chewbacca, which is where we see chewbacca putting C3PO back together.
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