The future of humanity is likely to be in orbitals. With existing materials these are likely to be 2-3 miles in diameter and 10-20 miles in length, producing EArth-like gravity by simply spinning. You then cover the surface with solar panels. If graphene ever becomes viable as a mega-engineering material, you can scale these up 100x+ larger. Why? Lots of advantages:
1. You can produce this with existing materials. No magic theorized materials are required;
2. No magical new technology is required. Even something like nuclear fusion power generation, which I'm not yet convinced will ever be commercially viable, isn't required;
3. Spin gravity is identical to normal gravity. Earthlike gravity means we don't have to worry about our ability to live in microgravity or even low-G (eg Moon, Mars);
4. You can largely be shielded from cosmic radiation with water tanks beneath your feet;
5. You get to harness the full output of the Sun for energy. For reference, only about one billionth of the energy of the Sun hits Earth. This is a Dyson Swarm;
6. This is incredibly efficient in creating living space per unit mass. Roughly 1% of Mercury's mass is enough to create a Dyson Swarm with billions of times of the living area of Earth.
So, by looking for technological signatures from exoplanet atmospheres we're essentially hoping to find the ~1000 year window where there are detectable signals and we're not looking for a Dyson Swarm instead. SETI has the same problem with a much smaller window.
So why is this much easier to detect? Because of thermodynamics. You absorb a star's light output and you heat up in anything less than perfect energy efficiency (which is impossible). The only way to dispose of heat in space is to radiate it away. Technically you could eject mass but then you have to replace that mass. And the frequency of the radiated heat is determined by the temperature of the radiating object. For any reasonable temperature, that's in the IR range.
The Sun is less than a million miles in diameter. If we were looking for a Dyson Swarm around a star like our Sun, we'd see an object that was more than 200 million miles across (ie the cloud of orbitals) that is producing much of that energy as infrared. It would stick out like a sore thumb.