JasonN said:
In my view, the Death Star only "worked" because seeing it at the end of EP3 was such a mindbendingly "HUH??!" moment that your brain had to mentally shut down in order to comprehend the logic behind it being there:

"So let me get this straight, Mr. Lucas... it takes the Empire 17-18 years in order to complete the first Death Star... yet it only takes them 2-3 years to rebuild it after Luke Skywalker destroys it... yeah George, that's complete bullsh*t."

but they hadnt finished the death star in ROTJ yet though
also its easier to build something when you have built it before
and if there was anything salvageable from the first death star they could use that for a foundation