They don't need to; it is the kernel that implements TCP, and on Linux at least you can easily change the congestion control algorithm that is used and some oth

They don't need to; it is the kernel that implements TCP, and on Linux at least you can easily change the congestion control algorithm that is used and some other parameters. Again, UDP doesn't magically solve anything, and QUIC implements congestion control in userspace, so then you have to wonder: is my QUIC implementation tuned for 8gbps over 1s rtt? Note that your example of file transfer is a special case that an application can be specifically tuned for; it can have a receive window that is as large as the file it is going to receive, because it can use the file on disk as the backing store for that window, and it can implement an unlimited form of selective ACK. This is really not something that you can do with a generic transport protocol that implements the POSIX sockets API.