It's just complete ignorance.
Here are some hard facts that many ATOT'ers would find hard to accept:
Music/dance conservatories such as Juilliard and Curtis are, by far, more demanding of their students than any engineering or science major.
Many students who choose careers in the arts and humanities have the intellectual ability to study a math-oriented field, if they wanted to. However, they choose to study their fields due to love for the subject matter, not for money. (One example is music... Most Ph.D. candidates in music theory produce dissertations that include extremely advanced concepts in mathematics.)
Likewise, many students who choose careers in the sciences would have weaknesses exposed if they were forced to study in a humanities major. The capacity for abstract thought and advanced communication are not generally prized in the scientific world.
Every college has its share of easy and difficult majors, but the level of committment required varies quite widely from school to school.
Quoted for truth. Sometimes, people have the intellectual capacity to do something, and they just don't like it. If it doesn't stimulate them or entertain them, they shouldn't do it.
That being said, it is much easier on the whole to produce some publishable-level BS work in the humanities. The subjective nature of the disciplines involved is generally responsible for that, as is the (sometimes) lax nature of those in the discipline who are more inclined to advance their own political ideology (be it conservative, or Marxist) than search for Truth.
However, I would argue that to compile a serious, scholarly work that would say, adress the rise of Modernity as a concept, and tease out the effects that that it has had on the last 150-200 years are so, including developments in science, religion, literature, the fine arts, philosophy, etc.--that would be a work that couldn't be replicated by most engineers, or any one else onthe planet.
In short, while it's much easier to BS in the humanities than in engineering, I'd argue that it's also harder to do serious, impressive work in the humanities.
Oh, and one more thing that may account for the difference. While engineers are rightly proud of the math and science classes that they've taken as preparation for their work, humanities majors aren't often required to take the classes that are, IMO, necessary for serious work in their discipline (e.g. 3-5 semester long, college-level courses in the following fields: Philosophy, Literature, Art History, Music, Religion, general History, and Economics; they should also have competence in at least one foreign language, and understand chemistry and math to a reasonable degree. For example, they should understand the concept of an integral, and know what integration does, even if they have to go to a book to look up how to integrate sin(x) + cos(x).