To make it easier for software to get a sane default? Same as why you put in your real name, it'll be used as default. For example I don't want to have to select my birth year on every website I visit. If there's a way for the browser to get a default year, then that's good for me.

Steam also asks you for your birth year all the time when visiting some of their pages, it's annoying as hell because you can't just type the year, you have to select it from a drop down list. They store it in a cookie, which is cool as it makes the process more streamlined. So you're also saying that this is a bad thing?

There is no law requiring you to put the correct birth date in a config file on your PC, the same as there is no law requiring you to write the correct birthdate on your wall calendar or whatever.