And 'Because everyone does' is not a sufficient answer.
It's a kilo-metre: 1,000 metres, that's what its name means - so why does everyone stress the OM.
And, if you do, why don't you say kil-OG-ram?
If anyone comes up with a convincing answer I may consider changing my ways (though not while my Dad is still alive, I don't think he's forgiven me yet for giving in to the majority and calling vinyl Vie-nil)