Sterilization is not a binary thing, it's a matter of degrees. All of NASA's Mars rovers have been constructed in clean rooms and sterilized according to planetary protection protocols. But there are levels of such protocols and though the levels that have been used so far are quite high they are not sufficient to reduce the chance of some micro-organisms surviving the trip to Mars low enough to justify risking putting the rovers in environments with potential Martian life.
Additionally, only recently have we been able to construct rovers that could host the equipment necessary to reliably detect and study Martian life, so the level of sterilization that we've been using hasn't been a problem.
Additionally, only recently have we been able to construct rovers that could host the equipment necessary to reliably detect and study Martian life, so the level of sterilization that we've been using hasn't been a problem.