In 1960, Freeman Dyson proposed how advanced civilizations could create megastructures that enclosed their system, allowing them to harness all of their star's energy and multiplying the habitable space they could occupy. In 2015, the astronomical community was intrigued when the star KIC 8462852 (aka Tabby's Star) began to dim inexplicably. While an analysis of the star's light curve in 2018 revealed that the dimming pattern was more characteristic of dust than a solid structure, Tabby's Star focused attention on the concept of megastructures and their associated technosignatures.
Because planets are terribly inefficient? Their livable area per mass is very low. You could deconstruct a planet and use the materials to make O’Neill cylinders (or similar) that can comfortably hold many orders of magnitude more people. It’s also very energy expensive to go up and down the gravity well of a planet, increasing the cost of all space-related industry, shipping and transport. For a interplanetary civilisation that’s a huge cost. Artificial habitats have no such problem.
It’s interesting the article focuses so much on Dyson spheres considering they’re probably the least practical megastructure. Dyson swarms are a much more practical and accepted concept, hell it’s one of the things that astrophysicists are actually looking for out there.