Because he doesn't know what he's talking about, and you shouldn't be taking camera advice from him.
The built in flash is perfectly fine for adding fill light to an image and with a bit of diffusion can even work as primary light source in a pinch.
The built in flash is perfectly fine for adding fill light to an image and with a bit of diffusion can even work as primary light source in a pinch.
And that's Greek to anyone who casually takes pictures.
Saying no one should use it is pretty extreme, but unless you have really poor lighting situation (nighttime outside or against the sunset), it's about 95% likely to produce a poorer image than without.
And this is advice for beginners who have no idea what a flash diffuser is.