Because grad school can be an extremely uncomfortable and lonely experience for some people. Academics are among the most self-centered and awkward people I know so it's easy to see how some people get jaded after a couple of years in their programs.
For some, it's the constant competition for resources, pressure to publish, teach, and have a life on the side that gets them. For others, it's the massive chasm between their romanticized notions of academia as this world of great minds versus the reality of their situation, which often consists of low income, depression, crazy work hours, anxiety, loneliness, and a host of other issues.
I have contemplated dropping out a couple of times in the past but it's way too late now, haha. And, I should also say that I do tend to overgeneralize when responding to these types of questions but I just wanted to give you a "view from the other side". Academia can also be a very rewarding and enjoyable experience. It's ultimately what you make of it!