The only alternative UI for many of these knobs is a linear gauge/slider. A knob is often modulated and automated so it's not just an input widget, it's also a gauge. It needs to convey an animated value to the user.
Typing is usually out of the question since you need to do it by ear. Some controls obviously have numberic input in combination with the knobs, for extra flexibility (e.g. if you made some notes with settings and just want to enter them).
Many of the values use mappings that make little sense as numbers. E.g. a knob marked 0..10 could mean a filter cutoff frequency from 20 to the nyquist frequency, logarithmically. Entering values in a text box with such arbitrary limits is a terrible experience (is it 20...22000...or 20..48000? or 0..1? or 0..100%?). Knobs hide the complexity of the logarithmic mapping and it's easy to to quickly set a value by ear.
In a few cases the knobs are decorative and could just as well have been e.g. most fixed step knobs could be combo boxes. For example a vintage synth often had a rotary knob to select between 3 options (waveform, for example) so now you can see that in ui's too. Doing that in a computer interface is purely for visual appeal.