Presumably because you (unlike the employer in this case) aren't a nasty person and would like to let your employer have time to hand over and tidy up tasks, hire or train a replacement and so on.
If it takes more than two to four weeks to transfer knowledge, both you and your management are doing it wrong.
You as employee should be cross training and knowledge sharing on an ongoing basis. If you are the only one that understands a process, that means you will be stuck on it forever. The person who puts themself in a position to never get fired also can’t be promoted. From a short term view, you also can never take a vacation without worrying about being called and interrupted because you are the only person that knows how the process works.
On an employer side, you should insist on knowledge sharing and cross training in case an employee “gets hit by the lottery bus”.
[0]https://www.erudit.org/fr/revues/rgd/2006-v36-n1-rgd01560/10...
Actually, yes, in europe they do. 90 days, contractually agreed upon. Unless there's a very serious issue like theft, violence or similar, you have 90 days until you're without work. Usually it is also at the end of a month, so sometimes it can be up to almost 4 months if they decide to fire you at the start of a month.
I also find it mildly amusing to make my last day at a company be on or just after April Fool's Day so my goodbye emails are funnier.