Why? Because lots of people like sending gifs and prefer to use messaging apps that support them. If it's totally useless to you then fine: don't use it.
It clutters the UI with unecessary stuff. Wire also provides such a misfeature. I would rather disable it, but cannot.
You have to specifically click a button to use it. That's a good compromise between jumping through hoops to enable it if opt-in.
In Signal is hidden under the post button, you can easily ignore it. In Wire it's on a toolbar and clutters the UI.
I think it would be acceptable if these types of things were on by default, and have the option to disable it or opt-out.
Yeah perhaps, but that would complicate the app development quite a bit. I'd rather see company invest developer time in other ways - maybe bringing desktop client support.