I'm from Spain and just use the old tricks transmitted through generations...

For boiling, if the egg sinks in water it's good, if it floats it's bad.

For frying or anything that needs breaking the egg, I break it carefully so that the yolk will not be broken by the hit, then I dip it into a plate. If the yolk keeps tight and nice the egg is good, if it breaks spontaneously or leaks some liquid, not good.

I've never had any problems and yes, usually they keep fine for a month without problems. I suppose the industry just prefers to be on the safe side with their recommendations.

In the case of frying, why isn't it good enough to do the float test?

It is probably good enough, but slightly more inconvenient considering how the eggs are prepared. It is pretty easy to slip an egg from a plate into a frying pan - some folks do this as standard practice. I was never sure of the origins until now.

If you are boiling, you are drawing some water anyway. It isn't a big deal at that point to have test water and add it into the pot when you are done.

Yep, that's the reason. Both tests would work fine but it's a matter of convenience.

Milk used to kill a lot of people yet many would drink it without issue for decades. So, your sample size is a little small in the grand scheme of things.

2-4 weeks seems reasonable. The eggs we got sometime this week (UK) have a Best Before date of 5th March (and the Best Before only indicates when they start to lose their quality - they remain safe to eat for some time after that).

OK, here's another sample.

The eggs I bought yesterday (I'm in the UK) have a "best before" date printed on them of 4 March, i.e. 2 weeks.

That is due to EU regulations. We used to have 4 weeks on eggs in Norway, but was forced to limit it to 2 weeks. Apparently it was all about salmonella risk, even though that is very rare in our eggs.

I've learnt something today. I don't think I have ever looked at the expiry date on eggs here in Norway and I've been here over thirty years! I definitely keep eggs for longer than two weeks unrefrigerated.