Because Zucker's contributions + 1.5 or 2M of spending on someone else would outweigh the sacrifice of spending 6M on Bertuzzi. It would give us a better chance of making the playoffs, at a time where we're underdogs to make it.
Zucker has good odds to be worth his AAV the next 2 seasons, which are the only ones we should be realistically concerned about. He has existing chemistry with Malkin, and he's one of our hardest workers. We have a pretty good sense of what to expect. Not the case with Bertuzzi at all.
Change for the sake of "not being lazy" is a ridiculous way to GM. You simply choose your best option, whether it's a returning player or a new one.
Subtracting a player who's already helping you in your weakest area doesn't make sense.
Plus for the people concerned about injuries with Zuck, Bertuzzi just missed 32 games last year, 14 the year before, and 47 the year before that.