Quite a lot of people who respond as you do actively don't want to understand. But in case you're one of the ones who is actually curious:

Historically, sexism has been a problem. For millennia, women were basically property. The last couple hundred years, we've been unwinding that. E.g. less than 100 years ago, women finally got to vote. They didn't have equal access to credit cards until 1974. In the 1990s, US states finally all made marital rape illegal. In the early aughts, women reached parity in the proportion of people getting medical and legal degrees. (The proportion of actual doctors and lawyers hasn't caught up yet; it's around 33% now, and I'd guess it will hit 50% about 2040.)

One place that sexism still resides is in language. That language at least sometimes shapes thinking. Many women in professional contexts reasonably object to gendered language that excludes them. So to be polite, many workplaces are shifting to use more inclusive language.

So, TL;DR: we're in the middle of overturning millennia of pervasive sexism, a language shift is part of that, and if we're lucky we'll be done with the project in only decades.