Ah, for the clean, crisp, well-defined world of digitals signals, with just ones and zeroes allowed, or maybe a few multilevel, multiphase signals. Too bad that once that signal is made real, in an actual circuit, it's subject to noise, distortion, and other nastiness.
But t gets worse. When the signal makes a transition from one circuit element, block, or subsystem to another, it's fully exposed to the real world and laws of physics. Sometimes, all it takes is a transistor with enough drive current, at sufficient voltage, to energize the next input in the signal chain. But often, it takes dedicated circuit functions of line drivers, receivers, and level translators, whether they are built into the larger signal-sourcing/receiving IC or implemented using separate drivers and receivers optimized for the physical path they face.
That's why the list of drivers and receivers, and the standards they meet is so long. These may be formal standards, such as the venerable RS-232/422/423/485 series, or internal, informal ones used between ICs, such as a simple 0/+1V interface.
Step back and make a complete list of all the physical-interface standards you are likely to encounter in a product's design, especially if it is more than a trivial circuit, and you'll see why it may seem like the real world is hostile to straightforward design. Guess what? It is.
Of course, every challenge is an opportunity–and in this case, the interface challenge is an opportunity for analog vendors and designers to be major participants in the design and BOM. ♦