Why can't I apply SPF50 sunscreen twice to get SPF100 protection?

These creams have some chemical that blocks the UV with some capacity, say a factor of 50. Why can't I apply two layers of this cream to now get a 100 factor equivalent protection?

I asked the chemist at the store and they said it's not how it works and that the highest protection factor they have is 75 (which was super expensive).

What gives?

Edit: Thank you for those super informative answers.