These games were designed to be played using a terminal interface.
The first video game about Terminal Interface was released in 1975.
Many early computers did not have a monitor or video output nor a keyboard port. They instead used a serial connection to transmit and recieve text with terminal. A terminal could be a "dumb" terminal that displayed text recieved from the computer, sent text based on keyboard input, maybe even transferred files between an attached storage device. Or it could be a "smart" terminal with it's own memory, own CPU, own data storage, have its own macros (even a macro programming language), and/or be "addressable", so the computer could tell the terminal, for examples, to clear the screen or display a score in the upper right or even where on screen to place text characters that represented player and enemies (instead of just placing one letter after another and moving to the next line like a typewriter). A full computer with it's own video display was sometimes used as a terminal for another. Typically a computer in a terminal role lacks something the other has (a better CPU, more memory, more storage, a higher price :)