I just saw an advert for Dogtown and the Dog Whisperer on National Geographic Channel. The byline was: “Every dog has their day.”
Well, that’s grammatically incorrect.
Pronouns must agree with their antecedent in terms of number.
“Their” is always plural.
“Each” is an adjective it does not increase the number in the noun that its describing
These sentences would be correct:
“Every dog has its day.”
“Every dog has his or her day.”
The answer to your question is two-fold, but simple.
First–no one cares about pronoun-antecedent agreement anymore, and the correct grammar sounds either stuffy or incorrect to most ears, so why bother…
Second–dogs have been elevated such that a dog cannot be referred to as an “it”; these are “fur children,” with rights and status in society concomitant with grammar proper for creatures with sophisticated feelings about their gender status!!! (Why not then use “his or her”? See the first reason.)
I can tell you this: I don’t have perfect grammar on this site, but when I hear blatantly incorrect grammar my hair stands on end. And keep in mind, you know in which state I live. Not surprisingly, my hair stands on end a lot.
“First–no one cares about pronoun-antecedent agreement anymore, and the correct grammar sounds either stuffy or incorrect to most ears, so why bother…”
I care. And it sounds terribly wrong to me.
Incorrect grammar drives me up the wall.
There may be some dogs who consider themselves gender neutral and might be offended by the “his or her”. Better to offend the grammar police. That’s the answer.
Thank you for not including an ‘ in the its. No doubt that if they didn’t use ‘their’ they would have used it’s. You know, because ‘ are totally genitive!
“The dogs have their day” would also be correct if the author is really committed to using “their”. :-)