Why is "lousy" pronounced like "louzy"? [duplicate]
As to why that sort of change happens in general: the /s/ in louse becomes intervocalic (it is now between vowels) in lousy and thus becomes more likely to trigger lenition. It adjusts to the sounds preceding and following it, which are both more sonorous and becomes voiced too. I think it's supposed to make the pronunciation slightly easier and is a common change observed in multiple languages.
For a historical perspective on this particular word, user067531's comment seems to be spot-on.