This has been discussed into the ground as one of the most popular topics on the Internet gun universe. Thus, I will expound. It has to due with risk and history.
1. Some organizations are scared of their gun carrying members. Thus, they maximize what it takes to get the gun into action to avoid an ND.
2. The Israelis supposed had many different guns so unchambered carry was the best training solution for them way back when.
3. For a civilian, you determine what risk is the one you are concerned with:
a. You won't get an unchambered gun into action fast enough because you have an arm disabled or involved with something else (child). I've carried with a disabled arm for a time and trained for one handed manipulation. They are a pain. Or you will futz the racking, as happens once in awhile, even if you train.
b. You are scared you will shoot yourself because you are a klutz or don't trust the gun not to go off if dropped or on its own. You also may think that an unchambered gun will give you an advantage in you are disarmed. The guy will futz with it while you unleash your gung-fu.
4. If you truly fret about this, carry a modern revolver.
That's it, culled from about 55,000 posts on the topic across the Internet.
