Apparently, the song "Happy Birthday" was copyrighted by a company. That's why you've never seen it sung in movies during birthday scenes. HOWEVER, it's copyright is ending on 14th March, 2016, due to a class action suit levied against the copyright holders. Might we want to celebrate that day with a Glorious Trainwreck event?

Here's where I learned the news:
http://nymag.com/daily/intelligencer/2016/02/happy-birthday-to-you-is-finally-free.html

...THEN I checked on Snopes just to make sure:
http://www.snopes.com/2015/09/22/happy-birthday-public-domain/