While birthdays are technically anniversaries; I've never met anyone who calls a birthday, an anniversary.

Birthdays ≠ Anniversaries. Just like Squares ≠ Rectangles. (All birthdays are anniversaries, but not all anniversaries are birthdays; just like all squares are rectangles, but not all rectangles are squares.)

I'd rather use the birthday field for birthdays. So when I say (and I imagine 99.999% of other Americans say) "anniversary" we mean everything other than birthdays (most commonly: wedding anniversaries).