Simple: plastic and aluminum expand and contract at different rates, and wear out at different rates. They're also cheap, I paid $100 over the counter for my last one and it was 15 min to change out (good piece of mind). These new rads aren't designed to last like the old copper ones, they're designed to break down and be replaced.
You can also pull your rad out and gently squeeze the tabs tight again, the locking ones that hold the plastic tank to the core. Its a simple rubber gasket in there so assuming that there's no debris in there is should seal back up. I also wouldn't recommend stop-leak in you engine either, yes it will seal up leaks but it can't distinguish between a pinhole leak and a small orifice in your heater core or waterpump. Conditioner probably isn't a bad idea, but regular flushes should take care of that anyways.