Why don't colleges make terms shorter and with fewer classes?
From an information processing perspective, it is much more beneficial to have four classes over ten weeks than two classes over five weeks. This is because the mind needs the extended time to digest the material that it is being exposed to.

The two class approach is highly beneficial in allowing students to focus. However, the reduction in exposure to the content over time makes it difficult for retention and to acquire in-depth mastery of the ideas of the curriculum. 

I have worked at several institutions that offer "intensive" courses for working graduate students. The students will take one class over the course of one week (about 8 hours a day). In such an intense format the students remember very little and demonstrate limited mastery of the content. This is due to the rushed nature of the course. Even though the students were able to focus, the average individual just cannot digest that much content so fast. 

Sadly, there is more to learning than just seat time. Students need time to discuss and apply the knowledge they are exposed to in a course. This only becomes more challenging as the number of weeks a course lasts is reduced, even if the number of hours stays the same.