I just assumed all female v-tubers were actually guys IRL. Is that just me?

No, it's a very common idea, though I'm not sure why. There are thousands of v-tubers, but it's the top 1% that make 90% of the money. Pretending to be a woman online isn't a cheat code to becoming rich and famous.

A lot of v-tubers are drawn to it because it creates a separation between their onscreen persona and their real life face.