Some newer synths do this where it makes sense. e.g. in Phase Plant the wavetable frame is a number, since wavetable positions are discrete values from 1 to 256.

Ultimately I see two problems though,

1. sometimes the number doesn't matter or make sense at all. A good example is a macro knob. The value is somewhere between "0" or "1", and synths do let you set it manually (since this is how recorded automation works), but a macro slider doesn't make too much sense IMO.

2. lots of controls deal with logarithmic values. Anything that corresponds to a frequency is going to need finer control when you're tweaking values below 500Hz vs changing a value between 10000Hz and 10500Hz. Knobs mask this pretty well. I'm sure you could build a slider that dealt with this, but a number box would be very weird since you'd want the scroll step to be much smaller at lower values.

Number boxes can be log or expo or even an arbitrary list, and they can have a fine tune through holding shift or the like. They also generally allow you to just type in the number you want. They definitely are not the best solution for all situations, just my preference.