Why build megastructures? Just move planets around to make habitable worlds

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.