Probably should have said, my sister did say something to them about it (I wasn't aware she had and hadn't said anything to her) and they seemed to be offended at the suggestion.
People have different standards as to what is an appropriate way to celebrate. Have your parents previously made a big deal about birthdays? If not, no reason to be surprised.
Do you have an otherwise good relationship with them?
If so, is it worth it to pursue the issue?
Some parents think their kids don't want a fuss made of them as they get older.
Don't sweat it. I wouldn't approach them directly about it, if you must - maybe get a sibling to sound them out later, if that's an option.