The problem though is that while serial is indeed much more commonplace than you might think (look at any device in your household, chances are high that it contains at least one internal serial port that was used for development), it’s all 3.3V or less with no negative voltages now. We don’t really use the RS-232 physical interface much anymore, it’s very unwieldy. (We also seldomly connect anything but the tx and rx lines, which is a bit of a shame for flow control, but often sufficient for what the ports are actually used for.)

So if you interface with those “modern” incarnations of serial ports today, your built in RS-232 COM port is useless most of the time anyway, and you already resort to a small, cheap USB serial adapter board that does the same thing at nowadays non-insane signal levels.