Probably the massive amount of money involved, and for very little gain. Most of them are going to crash hard and end up in the bottom of the ocean in little bits. Knowing where they crashed isn't going to save anybody, they're dead on impact.
But there's a lot to gain in analyzing the data. E.g. AF447 was a perfectly flyable plane that was lost due to bad piloting.
I don't know if Air France has actually started training and/or screening their pilots to help prevent a similar accident. But that's why we search so diligently; we need to know what went wrong so we can try to fix it.
AF447 Is a bad example they found bodies (first 2 were found within 5 days of the crash) and some wreckage pretty much immediately.
In this case the last known position wasn't "that off" our understanding of currents and ocean topography was.
Even if Radar will give you the exact point of impact with water (which it will never do) there are still so many variables that it might not be that much of a help to begin with over a general grid area.
Probably the massive amount of money involved, and for very little gain. Most of them are going to crash hard and end up in the bottom of the ocean in little bits. Knowing where they crashed isn't going to save anybody, they're dead on impact.