Why are guitar necks as wide as they are, who decided that that is what it should be? In guitar making there are standard neck widths. These have been worked out over time to be the most comfortable and effective. Much like violins, there are also very strict standard dimensions. For violin players this is more pertinent, because if you as a player, pick up an instrument that is not your own a this may be to test, or maybe an emergency before a concert, or a student that you are teaching a your muscle memory will automatically place your fingers in the right place to play the right notes. Now, if the dimensions are not correct, initially, you will have difficulty getting the right notes. After a while you will get used to it and you will sort out the difference, but if you move to another instrument you will have the same challenge. The same applies to guitar neck widths, there has to be a standard size. However, the guitar has frets which help tremendously in finding the position for the right notes and so, you do not rely on the same muscle memory as you do on the violin where there are no frets.
For the guitar, the smaller, or narrower the neck, the easier it is to play, the easier and quicker it is to change chords. But, it is also easier for your fingers to mute the string alongside the one you are pressing, the one you don't want to touch. So, if you have small, or average size hands, you will cope well with a narrower neck, but if you have bigger hands, or thicker fingers, you may prefer a slightly wider neck. The width difference in the neck to facilitate a change is very small, one or two millimetres would make a big difference! In this case, if makers and manufacturers made a variety of neck widths, their guitars would become a lot more specific and each instrument would be bought by someone who found that specific neck width comfortable, so they would to have to manufacture many more guitars in order to meet their sales targets. This would be good for the consumer, but not good for the manufacturer. One could have an instrument specifically made for you and then hope that the end result is to your satisfaction. You can also make your own instrument and when it comes to shaping the neck, you can customise the neck to your liking.
Having said all this, if you have an acoustic guitar with a standard neck width a steel string44mm (approx. 1,76inches), classical 50mm - 51mm (approx. 2,04 inches), after playing for a while, you will learn to cope with it and you will get to prefer your instrument to others.