It's far from being as simple as you suggest!
What we know is that Egypt gave a warning of something that wasn't especially clear of the nature of potential attack:
"We (Egypt) have warned them an explosion of the situation is coming, and very soon, and it would be big. But they underestimated such warnings," said the official, who spoke on condition of anonymity
www.bbc.com/news/world-middle-east-67082047
And:
"We know that Egypt has warned the Israelis three days prior that an event like this could happen," Mr McCaul told reporters following a closed-door intelligence briefing on Wednesday for lawmakers about the Middle East crisis, according to AFP news agency.
"I don't want to get too much into classified, but a warning was given," the Texas Republican added. "I think the question was at what level."
So, the threat level really wasn't clear. Israeli security services receive a LOT of reports of threats, but many of them aren't immediate threats. The challenge is always sorting out actual threats from perceived threats.
Apply the expanded Hanlon's Razor:
"Never attribute to bad intentions (e.g., malice or self-interest) that which is adequately explained by other causes (e.g., stupidity, ignorance, carelessness, incompetence, or lack of information)."
People commonly misunderstand the world of intelligence. Often, the assumption is that intelligence is always of high quality - wrong, it greatly varies from detailed information to very vague information. The other common assumption is that if an attack gets through, its deliberately allowed to happen which doesn't make much sense.
Often, contextual information is left out lest it undermine the conspiracy that intelligence allowed something to happen or to show they're incompetent. Other explanations exist such as incorrect information supplied, information not passed along the security chain, situations suddenly changed so intelligence agencies had to change plans last minute and many more possible explanations.
Intelligence and security failures can have multiple explanations as shown by the US enquiry into 9/11.
The following is an excellent point most people don't understand about intelligence:
"In fact, experts say the sheer quantity of intelligence that Israel collects on Hamas, as well as the group’s constant activity and organizing, may have played a role in obscuring plans for this particular attack amid the endless barrage of potentially credible threats."
And:
"“Intelligence in an environment like Israel isn't finding a needle in a haystack—it's finding the needle that will hurt you in a pile of needles,” Williams says. “Given the number of Hamas members involved in the invasion, it's not plausible to me that Israel missed every human intelligence reflection of the planning. But I feel confident that there are always Hamas operatives talking about credible plans to attack the IDF. So Israel can't respond with force to every threat, even every credible one. They'd be at a heightened state of alert or actively engaged all the time, and that's probably actually worse for security."
- Jake Williams, a former US National Security Agency hacker and current faculty member at the Institute for Applied Network Security
"The question everyone’s asking is, what role did Iran play? We don’t know. Iran has clearly been a supporter of Hamas financially, materially and politically. But we don’t know the extent to which Iran was involved in the logistical operational part of this training, or what kind of logistical support (it offered the October 7 operation).
I don’t think anyone knows that. Every (country’s) intelligence was caught completely unaware of this, including and especially the Israelis."
• Khaled Elgindy, senior fellow at the DC-based Middle East Institute where he directs the program on Palestine and Israeli-Palestinian Affairs.
On top of that, see the attachment where Hamas boasts they fooled Israel and the West into thinking they were busy governing Gaza Strip.
The lesson learned here is that receiving too much intelligence on potential or actual threats is a security weakness and that intelligence services are NOT infallible although they are an important part of the national security apparatus.