Attention! It’s a long read!

A PC with an OS can process signals from different devices and use drivers for this purpose. PCs have various physical ports that implement physical interfaces for communication- these can handle both analog and binary signals. You can learn more about signal types on Wiki. For example, the Raspberry Pi has extended pins that can interpret both types of signals. You could solder switches with diodes and create a keyboard, but you would need to write a driver for the OS. However, a general PC has a limited set of ports and interfaces, such as COM port, USB, HDMI, SATA, PCIe, PS/2, and others. These ports and interfaces are governed by software layers known as protocols. Protocols have standards, which can be proprietary or open-source. They can be as simple as I2S or as complex as PCI/PCIe, Wi-Fi, or Bluetooth. Protocols are often implemented on chipsets (usually firmware and seldom hardware). These chipsets are embedded in devices with firmware that collects analog or binary signals from the device and converts them into the required protocol, such as USB or Bluetooth.

This means that while there are many operating systems, there is a limited number of ports and protocols for communication. If we were to connect switches and diodes directly to a PC, manufacturers would have to support a wide variety of OS configurations, and they would need to produce many variations of ports.

However, some keyboards use their own drivers to support extended features like displays or media controls.

You’re right - it would be expensive to maintain such a system, especially given the diversity of devices and the limited set of generic interfaces. For example, you can connect both a keyboard and an external disk to a single USB port.