Why is so much music recorded out of key?

Lots of things may happen when music is committed to recording, not least of which is that the players were not tuned to a perfect 440 or whatever to begin with.
There’s no such thing as being “off key,” only “off pitch” which refers to an instrument or vocal not being in pitch with whatever frequency the music is tuned to. There’s no law that says we have to be at 440. One may prefer something different (likely a little lower…there’s a whole thing with 432 Hz)

Doesn't bother me and I doubt I'd be able to tell; these are not quizzes or tests, just music that one either enjoys or does not. 

Hmm: having @fuzztone answer this particular question has a certain Je ne sais quoi….

Check out pages 55,56 of the book God, No! for a rational take on the subject.

It's more important that the musicians are in tune with each than with concert tuning.  Analog recording and manufacturing had numerous places where small errors in pitch could occur.  The biggest selling jazz album of all time, "Kind Of Blue", had pitch/speed issues.  I can imagine for someone with perfect pitch that it could be annoying, but for most it doesn't matter.  With digital recordings you can intentionally pitch up or down to your heart's content.  It's just another FX tool.

Google says that the 440 hz A was established by the American Standards Association in 1936. In other words, the choice is perhaps a bit arbitrary. But it always seemed to be somewhere in that neighborhood.

@alvinnir2 

Not sure exactly how it got started but that is what I had heard when I first asked the question. When I first started learning I always tuned the guitar to my folks piano only to walk back to my room, put on a record to play along with and found that I was out of tune. I don’t think it has to make sense- it’s just what it is.

Interesting comments.

@designsfx Doesn't make sense to me to tune down a quarter tone for the vocalist as a quarter tone difference shouldn't be that hard for a vocalist to deal with ,even at the top or bottom of their ranges. Also this phenomena occurs with recorded music that is all instrumental as well. I suspect bdp24 is right in that the producers do it for some reason. 

When I was younger I played guitar constantly- rarely was A440 tuning used. Most rock guitar players followed the 1/4 step down tuning- others standard. It was all about the range of the vocalists. There are many others as well- Drop D and open tunings as well as the move to 7 string instruments with offsets on 6th and 7th. It just takes a trained ear to catch what’s going on.

That auto tune garbage

it makes the music match the voice or something.

years ago, real musicians did t have this issue.

these new bands and singers are all idiots, and need autotune to sound average

check YouTube for that Brad Delp singing without,music to his Boston song  more than a feeling, that was all him, a real singer. These days. Blechk!

all junk, minus a few of course

running slow...then slowed down

OK What does it mean?

I do agree with blaming producers. A half tone off makes their "creation" stand out from the pack in search of the hit.

Some singers are recorded with the tape machine running slow, to lower the pitch of a note above the top of the singer's range. In post-production the tape is then slowed down to return the song to it's original pitch and tempo. In addition, some producers try different running speeds, to find the "feel" they are looking for in a particular song. 

Here's some information.