There is a difference between a terminal and a shell. When you see a pts window, there are both a terminal-emulator (pts) and a shell (bash) running there. Bash is reading lines from the pts device and executes commands. Bash writes its stdout/stderr back into the pts device, and the programs started by bash do so too. But the pts itself is just a glorified serial terminal. It displays characters written into it, and you (bash) can read characters typed into it. Usually it also echoes (displays) the typed characters.
When you write into a pts device from another terminal, it displays the characters, but these characters cannot be read from the pts device. You (bash) can only read from the pts what a user types.
The confusing thing is that displaying characters written into the pts device (this is what you tried) and echoing the typed characters look exactly the same.