Zero-copy network transmission with io_uring

It's a matter of application latency and reasoning about how the application behaves. If the sent data is small enough you can get better application performance by copying the data into the kernel and giving the application the completed signal, then let the buffer work through the network stack at the kernel's pace.
There is also a problem in that the logic gets a lot harder if the application has to manage separate "message send begin" and "message send complete" signals. People have a hard enough time writing network programs that don't deadlock and zero copy makes it more difficult. So it makes a lot of sense to write an API where the simple and easy case is default, and if they need the power tools they have to ask.