Because Mossad is one of the best in the world and they were reporting of suspicions Hamas activity. An Egyptian official also warned Israel of incoming Hamas attack. The attack took years to prepare.

"The groups carried out joint drills in Gaza which closely resembled the tactics used during the deadly assault - including at a site less than 1km (0.6 miles) from the barrier with Israel - and posted them on social media.

They practised hostage-taking, raiding compounds and breaching Israel's defences during these exercises, the last of which was held just 25 days before the attack."

Well, look at Gaza. If you are Israel, you hate that it exists (and of course, vice versa).

In 2006, Gaza had the borders closed and was sealed in. Gaza's population was 473,257 people. Instead of giving in, Gaza decided to have a lot of children, resulting in a population of 2.048 million people as of 2020.

Kind of a failure on multiple levels. And now all those people want to kill you for what you did to them. That sucks. Your prime minister who has served for over 16 years is taking the blame with about ~20% popularity. Things need to be fixed.

One day, you wake up, and find out Gaza's planning an attack. And for once, instead of responding forcefully, you decide: What if we did nothing? They attack, we respond with the world behind our backs, and solve this problem.

I'm not saying that what's happened. It just seems plausible.

I don't buy into the grander conspiracy, but it does seem increasingly likely Israeli leadership "knew about it." It has been now well-reported that Israeli intelligence got multiple warnings both from their own SIGINT and from Egypt. They essentially had the blueprint for the plans a year ago, they observed a huge training exercise that was identical to the plan a few months ago.

Now the problem with retrospectively "knowing about things" is that it's hard to account for the thousand other threats these people are constantly being made aware of and which ultimately don't materialize. Or at least I presume that's the case.