It's not surprising that it wasn't seen until it was effectively stumbled over. Dyson spheres would not be particularly easy to see, even if you're a couple of lightyears away – especially if you're not looking for them.
First – while enormous in absolute terms, the mass of a Dyson shell is spread out over its entire surface area, and it's still a tiny fraction the mass of the central star. The "mass" of a Dyson sphere when you're next to it in space would be negligible. Gravitationally, being next to a Dyson shell with a diameter of 200 million km would be almost indistinguishable to just being 100 million km away from a regular star.
Second – by design a Dyson sphere intercepts radiation from the central star; and a full Dyson shell would intercept 100% of that radiation. The only detectable emissions would be the waste heat of the Dyson shell itself; it would radiate mainly in the far infrared, and not be terribly reflective. It would, in other words, be
very dark. They would be virtually invisible to conventional telescopes. In fact there are projects today to look for Dyson spheres in the galaxy
specifically by looking for unusual far infrared sources.
Third – it's also worth considering that supergiant stars are far larger than a typical Dyson shell, and they still appear point-like to us. For example, the star Betelgeuse in the constellation of Orion has a diameter of around 1.2
billion km, versus the "Relics" Dyson shell diameter of only 200 million km. Betelgeuse is approximately 600 lightyears away, and while it's so big that we can resolve its disc with our most powerful telescopes, it's also much,
much brighter than a Dyson sphere would be.