Okay, then why not just take the /dev/ttyACM0 output and redirect it to a standard TCP port with like 5 lines of code?

And can't multiple applications read a /dev/tty device read-only with just e.g. "cat /dev/ttyACM0"?

Technically you can (in that you won't usually get an error), but the result is not what you would want: each byte only gets delivered to one of the readers, in a somewhat unpredictable manner, either mangling the data or starving one of the readers

No, multiple applications cannot read the same data from a tty device.