I'll tell you the real reason guitarists can't read music. They don't do it enough. Most guitarist get started with nothing or with tab and develop the skill to get by with poor reading skills. I was a jazz major for a year when I got out of high school, plenty of guitarist around who could site read just as people who played other instruments. If you've seen a twenty something guy sit down and do a decent first read of Eternal Triangle on guitar than you know it can be done.
I disagree. I learned piano as a kid, and then taught myself guitar as a teenager, so by the time I picked up a guitar I had already been sight reading on piano for 5+ years, and additionally I had very little access to guitar tabs at the time because this was before my family had internet access, so a lot of the material I learned early on was from piano-centric sources, typically sheet music, occasionally with some guitar chord charts thrown in. So if the problem is that too many guitarists are exposed to tabs first and then are too lazy to learn to read sheet music, I should be immune to it. I knew how to read sheet music before I ever touched a guitar, and from the start I was attempting to learn guitar by reading sheet music on it. Yet to this day, I find tabs far quicker and more intuitive to read than sheet music, when playing guitar. Even if I use a score that has both sheet music and tabs side-by-side, I find myself primarily relying on the tabs, and only glancing at the sheet music if I need a clarification on timing. Now I definitely agree with you that it's possible for a guitarist to learn to read sheet music quite well, but it takes a lot of work, far more so than what it takes for a pianist to learn it, and it's because of the issue mentioned in the article: sheet music notation maps nicely to the keyboard, but not the fretboard. That's why there are plenty of mediocre, amateur pianists out there who can sight read competently, and many excellent, sometimes even professional guitarists who struggle with it. The reason guitarists use tabs is because of the nature of the instrument, not the nature of the guitarists.