I do get it, but you apparently do not. The individual terrorist organizations are pretty small and independent, and are basically defined by their leaders. Remove the leader, and the organization falls apart. This is, BTW, a good way to apply direct and massive force to the terrorists without inflaming the rest of the nice folks living in the PA (who, BTW, outnumber tie Israelis).
Killing the leaders is also an effective reminder to all who would step into the shoes of the dear departed that they, too, will be killed.
It's not possible to kill them all at once, of course, because they're never in the same place at the same time.
There are still ways, though. Israel's springtime invasion allowed them to target multiple bad guys near-simultaneously, and I guess they got quite a few of them. But their ability to continue the campaign had to be weighed against the possible loss of foreign support that that invasion provoked. (For Israel, please note, PR is a vital national security issue.)
I do see that they've been continually taking out one or two bad guys every couple of weeks. That's actually a pretty rapid pace, but spread out enough to allow the Israelis to avoid the bad PR that would accompany a concentrated and short-duration extermination effort.
The downside to this approach is that it does expose civilians to danger. But the alternative probably puts them in even greater danger.
Beyond that, killing the terrorist leaders will also probably have a greater long-term payoff than would creating a bunch of Palestinian "martyrs." For the former, the driving force behind terrorism is systematically taken away. As for the latter, you've just created a whole new batch of reasons for kids to strap bombs to their bellies.
Now it's your turn. What exactly do you mean by "direct and massive force?" Against whom? And to what end?