Because you choose when your end date is, not your current employer.
It's polite to give at least two weeks notice, but to meet that social norm you just need to only agree to start dates at least 2 weeks into the future. (And if you want to give more than 2 weeks notice... well... that just means you need to shift your start date farther).
It's a social norm that needs to die, since it's no longer reciprocated. It's from a time where a company would typically give you some severance if they had to lay you off. Now they just walk you out and stop your pay the same day you're laid off, so I'd be perfectly morally and ethically okay with doing the same to them were I to quit. Granted, I'd only do that if I didn't like my current employer.
My personal opinion is that it should depend on the situation.
If you leaving suddenly wouldn't cost society any more than you getting sick suddenly, I don't think the norm does any good. That's true of jobs where you're a replaceable cog (like a clerk at a store) with no need to transfer knowledge.
On the other hand if you're at a job where you have long lasting context, which it is painful to lose, it would be bad for society to have it be the norm that you quit without transferring that knowledge.
I don't think that this norm needs to be tied to the norm about how you're compensated (provided that you are fairly compensated, but if you're not, you shouldn't have taken the job in the first place), whether that compensation includes notice when you're being fired, or not, seems irrelevant to whether or not it should be considered your responsibility to give notice. The two situations are only superficially similar.
Is it? I have a notice period of three months, so I figure if I were to try and switch jobs I would have to tell my future employer that I couldn't start any sooner than that. Else, I am potentially liable for any costs my current employer has due to having to replace me inside of those three months.
Yes, two weeks is pretty normal (in the US and Canada), but it's also normally not written into your contract, just a social convention.
I imagine your contract is binding (though I'm no lawyer and it may depend jurisdiction), it's just not a common term in my experience/the experience of people I know.
Personally, yes, I think I would. Or really somewhere between 1 and 3 depending on how long I thought I really needed to facilitate an orderly transfer of my work to the rest of the team. Winning the lottery is a case where I would have a ton of personal flexibility, and I'd like to think that I'd use some of it to not put people in uncomfortable positions.
But I also think this misses the point, plenty of people do plenty of rude things when they win the lottery (so it's not a good barometer for what is polite), and the circumstances are substantially different when you're leaving because you now have "f u money" rather than leaving because you're going to do the same thing for a different employer (so it's not a analogous situation).
Circumstances where I wouldn't give advanced notice are cases where I felt I was being mistreated (being rude is worth leaving earlier, plus I have minimal sympathy), or there was an opportunity that really couldn't wait (being rude is sometimes worthwhile - it's a balancing act).
I mean, assuming I have a pretty normal amicable relationship with my team, manager, and employer ofcourse I would. 2 weeks probably isn't enough to employ a replacement but it's usually more than enough to hand off key functions, transfer knowledge, give people time to pick your brain and honestly - have you not had goodbye parties?
I’ve pushed start dates before when there was an important customer facing project and a new contractor to get up to speed. Instead of 2 weeks notice I gave 3. Any future employer who wouldn’t agree to that is not someone I would work for.
I’ll join on 1st November because that is when I’m available. Assuming you’re in America you can give your 2 week notice anytime between Oct1 and oct15.
max(<how much time you'd like to stay at the current employer> , <your legal obligation to your current employer>) + <vacation time> + 1 week for slack
It's polite to give at least two weeks notice, but to meet that social norm you just need to only agree to start dates at least 2 weeks into the future. (And if you want to give more than 2 weeks notice... well... that just means you need to shift your start date farther).