Because if you don't, someone who does is going to invade for energy and resources. Therefore, refusing to expand is a slow form of suicide, unless you can be somehow certain there's no entity like that in the potentially accessible space.

At some point growth in any star system is going to be finished - energy extraction near maximum, no free matter left to convert into computronium. After that, only two solutions remain: stagnation (death) or expansion. Assuming no instant communication (not just FTL - instant), the bandwidth argument only means it makes more sense to haul energy and computronium from the rest of the accessible space into one point.

We may very well already see it somewhere far away, thinking it's "natural" rather than a construct. I think under current knowledge the most likely prediction is something resembling a sphere (minimizing distances between points) that utilizes black hole power plants, feeding them matter from stars and nebulae.

From the outside, it would look like a giant void with nothing in the center. Maybe there's something like that in the center of the Boötes void?