By indwelling these complex human sensor mechanisms (like putting on a kind of suit if you will), aliens can not only see, hear, feel, and smell this planet, they can comprehend it with a vast array of emotions, drives/ambitions, fears, loves/hates, loyalties, betrayals, etc. And don't forget the sexual experience, which BTW seems to dominate much of the alien abduction experience.
As you mention, the aliens' lack of emotion makes this world seem flat to them. It is human emotions that bring it alive...and that is what they want. They want to co-exist inside of us so that they can share our passions (including compassion, sympathy, empathy) for life.
They want something we have that they don't have...life itself.
The human soul is anchored in the heart to give life to our physical bodies. It is where the masculine and feminine aspects of consciousness connect when two souls are in a relationship, and the reason for the heartache when they part.
The whole reason the weeds started Covid-19 through the CCP was to attack people’s hearts and split this connection, effectively taking away access to their logical aspect so they could move in and attach.