That's a good point, Jiminy - though thinking of one particular male friend, it was probably very clean because he paid a cleaner and lived on oven meals 
It does seem to be more in a relationship that things change and the share slides. At least going from what friends complain about, and people post on here. Is it men giving up, or women taking over? I appreciate once you get into the realms of having a SAHP then things can change quite quickly and dramatically and that's potentially a much bigger discussion (i.e. relating to age of DC, DC's personality etc). But why do some men give up completely? Why do some women take over?
I have a sneaky suspicion, which is little more than a gut feeling over 5+ years of reading MN, that...
1 - A lot of men of a slightly older generation went from having their mum do everything, to having their wife do everything, and so just passed the buck
2 - A lot of men get very, very quickly used to having a SAHP and decide that if they are earning all the money, the least their partner can do to "earn" their money is doing everything at home
3 - A lot of women have somehow been brought up to accept that they start taking over this stuff.
I know there's no real answer to all of this, in terms of where it comes from. It must be a mix of societal upbringing and expectation.
It depresses me when you read it so often on MN - "my husband is so incompetent, he can't do a thing..." I want to say, no, you're letting him get away with it. And isn't the onus then as much on women to say - sod off, love, and pull your own weight? Isn't there more there that is teaching women to do it, than teaching men not to do it?