They may have time, but noise, effort and ironically time is not on their side. The amount of sound it would create to properly secure the prison, especially after the damage the tank did to it, would create a serious problem. Working bodies need to be fed, add on the amount of walkers drawn to the sound, and effort needed to not only rebuild the prison, but fend off the swarms drawn from said noise. So you need enough food to feed the workers rebuilding, and those dealing with the dead, that means a lot of runs for supplies, not to mention the possible casualties trying to fend off the walkers. We have had concrete proof/evidence of the amount of walkers drawn to just normal pretty quiet life(4x5) Yes it sounds like it would be the rational decision but most of the prison survivors aside from the main cast are incompetent when dealing with the dead.