I wish I knew…and this is a bigger question than you think you’re asking. When we count we go 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and then we start over again, just changing the second number, 11, 12, 13…etc. This is called “base 10”. The base is the number that you have to hit before moving a decimal place over. We use base ten, presumably, 100% because we have ten fingers.
However, 12 is possibly a better choice. Ten is only divisible by 1, 2, 5, and 10 while 12 is divisible by 1, 2, 3, 4, 6, and 12. So for a lot of applications, base 12 is easier to use. And we do use base 12, just not very often or very precisely. Every time you say “two dozen” you’re using base 12. Or, in the case of your pack of tampons, eight dozen.
Why we use dozens isn’t exactly clear…it may be just because it’s mathematically convenient…or it may be good for marketing reasons (96 might sound more impressive than 100.) Or maybe it’s because there are roughly 12 lunar cycles per year (which is where we get the 12 months.)
We don’t really know…but beer, soda, eggs, and tampons…all come in dozens…for reasons that stretch back, possibly, to the very beginning of counting. Which is REALLY COOL.