I don’t have quantitative evidence for this, but based on conversations with others at various points, from my school days onwards, the most underdeveloped of all musical skills in those who have taken formal lessons is sight reading.
The reasons this might be so are largely because of the way in which formal music is taught. Typically you will practice three pieces for around six to nine months, offering them for review in an examination. The examiner will mark  based on time, tone, shape and performance. This translates into accurately rendered playing with due observation of dynamics and an overall ‘bringing to life’ of the piece — in short as close to perfection as you can manage. The way this is brought about is by practicing over and over again until the best result achievable is achieved.
During that time you encounter only the music you are playing — a relatively small number of pages which you will read initially, but which will serve increasingly as a prompt and eventually, if repeated enough, will become unnecessary, the piece having been committed to memory.
It is also worth noting that at no point in the process will you have read music with a view to performing it accurately there and then — a statement which has to be true when you consider the fact that the examination pieces will almost certainly at the time be at your own personal technical limits — you cannot develop if you constantly play well within your comfort zone.
Practical music examinations do usually incorporate a sight reading component which the student is required to pass and sight reading books are available for each grade, but they generally contain a limited amount of music— quickly worked through. Sight reading demands sufficient unread music and without it you cannot progress - it seems there is not enough to allow the skill to be adequately acquired.
Or is there …
Around ten years ago I was privileged to have an incredible piano teacher. He could tell without looking at my hands what fingering I was using and he was extraordinarily good at giving me technical exercises to improve my ability to play passages I was struggling with. On sight reading he gave me some advice which although dispensed in a few seconds gave me food for thought for the rest of my life. He said
‘First play all the Mozart Sonatas. When you’ve finished that, play all the Bach Preludes and Fugues. When you’ve done that play all the Beethoven Sonatas. Once you’ve finished that, you’ll be good at sight reading’.
Now that might seem like laughable advice. All in, that’s probably around 1400 pages of music. How could you fail to be a good sight reader if you did that? Additionally, how long would that take — how about some advice that is achievable in a foreseeable time frame?
Well that’s the point. You are going to be a lot, lot better than when you started, regardless of the level you start from. In answer to the second question, if you were to play a page a day it would take you nearly four years, which is a timeframe too far out for most. But, after one year you’d have radically improved and what the advice does is set out a path for you to follow. Of course you don’t have to follow the plan exactly — you can do this with any suitable level (for you) big book of music. The point is, you can just pick up and play, and if you do this systematically and for long enough, you will get a lot, lot better.
You can make the case that sight reading is the most useful of all musical skills. The better it is, the more your musical world opens up. If you can’t sight read, your only way of playing music is to painstakingly acquire. You will never be able to just ‘sit down and play’ — the thing that amateur musicians and non-musicians alike aspire to do.
Sight reading increases your musical education. The more music you play, the better understanding of it you will have. You learn to recognise structure, harmony, modulation, melodic direction, rhythm and style. Along the way you will find new things you like. You will get a sense of the composer’s trajectory if following the work of a single artist.
It may seem the bar is being set a little high. For most, that music will be technically challenging even at the easiest end. There are few who will successfully a Bach Fugue at first pass. Many Beethoven sonatas are successfully played only by those of a professional standard. The point is however, it can all be played taken one note at a time and the more you do it, the easier it will get. Set your sights high and you will be surprised.
So what did I do? I did get through all the Mozart and half the Bach. I’ve done 3/4 of the Beethoven and will probably get to the end in a year’s time. I do something slightly different now — I play 3–4 pages for a week before moving onto the next 3–4. The music’s just too hard to be decent at a first go and it’s a journey where it seems pointless to not enjoy the scenery. I’m definitely sight-reading — can’t play without the music, but giving it slightly more time allows me to gain more from it than a first pass allows.
I have got better though. It’s something everyone can do, few will. It will pick you out if you do.