If that was the case then no wrestler would wrestle in the build up to an PPV and house shows wouldn't exist.
CM Punk is wrestling, and Punk is also needed for the match. He could get *re*injured during a match.
Rock isn't wrestling because they don't have to have him wrestle. He didn't wrestle prior to his match with Cena bar the tag match at Survivor Series 5 months earlier.
Punk-Rock mic segments are probably going to build the match better than having the two compete anyway, especially of last week's RAW is the benchmark.