Because gifs can't have volume. I don't want my phone to make noises when I'm browsing the web.
Also probably more addressable / available tools for anim gif creation.
Don't blame a web video format/standard for UI failures.
I think you don't understand the modern web. If something is crappy the user shall have it. JS was always considered crappy but now is the best new thing. And you also missed tbe latest web/UI/UX development. From the web designer point of view the user is an idiot which must be punished for every attempt to customize something.
> Don't blame a web video format/standard for UI failures.
They are not UI failures. They are done on purpose.