My guess is that assassination isn’t as easy as it is made out to be in the movies. The CIA, the best funded intelligence agency in the world, tried to take out Castro hundreds of times and failed. They couldn’t find Osama for a decade, either, and even then the US used Seals, not the CIA. Sure, killing some rando is probably easy, but not a government leader who is actively avoiding assassination, as I’m sure Hamas leaders are doing.