Why cant Israel just take out President Ahmadnejad?

Because Iran is so powerful and Israel knows it. It's kind of playing with african bees. In our today's world, diplomacy is the best way to solve problems with countries that are powerful. Didn't you learn anything about the mess created by Bush when he invaded Iraq ? It's a war impossible to win, even for the USA as you can see, and Iraq isn't Iran. Another point here is that Israel hasn't the same support from the USA as it was with Bush, it's not unconditional support anymore.