Because we think intelligent alien life has nothing better to do than destroy the natural beauty of star systems in order to power their crypto-coin mining rigs and video game cards. ;-)
Edit to resolve Poe's law ambiguity: this is a parody. Obviously aliens would have lightning network which would require a smaller overall dyson sphere
So you think an alien civilization advanced enough to build a dyson sphere would not know how to leverage it for the fundamental goal of implementing blockchain?
I guess it's true what they say: there's a sucker born every minute.