This is actually not true. Gender has been very fluid across cultures and time periods. There have simply been time periods in which thinking was rigid and dogmatic, and the 19th and early 20th century was one of those.
This is not a linear progression, it's a sine wave.
Fine, but it's still longer than the last couple of hundred years, and doesn't really change my point. We do this stupid "gender reveal" shit because gender is extremely important to most cultures today (and for at least the past several hundred years).