My pet theory is they're already here and don't care at all about environmental issues. Planets like this are a dime-a-dozen to them.

However, nowhere else in the universe have they come across music like ours. They are madly obsessed with it and dare not intervene in our affairs, lest they taint the source.

I have absolutely no evidence for this. I just like the idea that Freddie Mercury just might have been an intergalactic superstar.