Because early reflections are perceived as the same sound as the original source and therefore enhances what you might call the "spaciousness" of the sound. This only works well, however, when the off-axis sound that's getting reflected is substantially similar to the on-axis, hence wanting constant (or at least smooth) directivity. If the on-axis and off-axis don't match well due to poor directivity, then the reflections end up coloring the perceived sound. There's also the fact that absorbers generally act essentially as low-pass filters of varying frequency depending on their material and thickness, and it's easy to actually cause issues by misapplying and/or overapplying them.
Note that there are those who disagree with this, although studies show a listener preference for early reflections by my understanding (again, as long as they aren't unduly coloring the perceived sound). Rather than applying absorbers everywhere (though you certainly want some to tamp down liveliness, but often standard furnishings like carpet/rug and furniture achieve that goal just fine on their own), better to just get a speaker with good directivity in the first place IMO. Cardioid speakers (like the Dutch & Dutch 8c) or placing quality speakers into the wall would be my preference.
If I've made any errors here, I'm sure someone with more expertise will be along to correct me.