On the contrary, introducing their own novel/mysterious/poorly implemented congestion control algorithms is not a thing I want userspace applications doing.
And if you worry about hostile applications on your own hardware, the OS is an excellent point to limit what they can do – including overwhelming your network interface.
One might even call this "congestion control"!
While you're still on your own host, your OS has complete visibility into which applications are currently intending to send data and can schedule them much more directly.