Why don't you ever see a "anything" over (cable type)?
To expand slightly on the obvious "because it's very hard / expensive" answers:

If you know what you're sending, say some digital signal that has a defined form - for argument, say it is some digital data like SPI which may be up to 1MHz string of 1's and 0's. That is easy to convert because you know a 1 or 0 can only arrive at a certain time and last a certain period, and the receiver knows the same and can easily clock the data back out with the correct timing, level, etc.

If you have an "anything" converter and you want to be able to send your stream of 1's and 0's at ~1MHz, you must sample it MUCH faster (maybe 10x or more) to accurately reproduce the waveform at the other end. You also need to accurately measure the amplitude, as you no longer know that 0v is 0 and 3.3v is 1, so maybe 1.5v is important too, maybe 33v is, so you have gone from transmitting 1 million bits per second to maybe 10 million multi-bit measurements, many orders of magnitude greater.

If you look at data conversion/transmission operations like sampling &amp; encoding you can fairly quickly prove this to yourself, for example nyquist frequency if you have to accurately reproduce your 1MHz stream of 1's and 0's by sampling rather than being able to rely on the timing of the bits.

Edit to add: An example you can easily try at home; Open a text file, type "hello world" and save it. How big is the file? About 11 bytes hopefully. Now take a screenshot of that text and save it as a full-colour bitmap. How big is it? Waaaaay more than 11 bytes. Why? Because the picture can't know it only contains 11 bytes of information, it is storing the colours of hundreds of pixels at probably 3 bytes per pixel.