Yes, engineering is hard. It is much easier to slack through a liberal arts degree, yes. However, for me I felt that my education in computer science was too narrow. To study it I had to drop jazz band, I had to drop American lit, I had to drop film, I had to drop art, all things I was passionate about in high school (in addition to writing code).
I didn't work any less hard in my easier major. In fact I probably worked harder. A lot of my friends who also defected felt a similar sense of narrow scope in studying engineering.
While I do think a lot people drop engineering because it's hard, I think that a lot of people who are able and intelligent leave STEM fields because the fields themselves do nothing to try and encourage its students to develop outside interests. And those outside interests are important, they can be life changing.
Similarly, the liberal arts majors do NOT do enough to encourage its students to explore STEM subjects, which is just as horrible, in my opinion.