Even if technically possible, space travel is expensive.
In a post-singularity society, energy and mass can be highly efficiently used to generate their best approximation of computronium -- specialized states of matter that compute highly efficiently.
To travel over interstellar distances, you need a lot of energy. Energy that would otherwise be sufficient to generate a rather huge amount of computation. Imagine if our solar system had 10^9 planets like Earth, all fully populated. And in order to send 3 people to a nearby star, we'd have to destroy 10% of them and kill all the people on them.
In the long run, such a probe would result in a larger total "civilization". But with bandwidth restrictions of interstellar communications, those 3 people would be sent off, and would only arrive the equivalent of millions of years later (faster time scale from high clock speed), with the ability to send single bit of data every thousand years (again, scaled based on intelligence of us vs them, and how much they are used to communicating). So even once they are there, they are going to be sending messages back to a really alien society, and there isn't enough bandwidth to discuss what is going on (at the level the civilization is used to), let alone coordinate.
So now you have a society that destroyed 10% of its population, and in exchange has created a colony that can not meaningfully interact back with them.
The upside might be hard to see.
Those 3 colonists arrive in a barren wasteland. Suppose they can manage 0.1% economic and population growth per year until they hit the equivalent of 10^20 people (they are building an economic system from scratch). So it takes them all of 50,000 years to reach the previous point of a civilized star system. At which point they could go and kill off 10% of their society (or prevent them from ever existing) and launch another star wisp.
Repeated, this does colonize the galaxy (10 ly every 50k y times 100k ly is only half a billion years. If the trips take another 50k years, that only doubles things) (so long as your star wisp production per star wisp launch stays detectably greater than 1, the limit is distance: the wave of colonists ends up being limited by the geometry of the milky wa more than anything).
So the capabilities of a post-singularity society might be great, but the other things they can do with those resources is equally great.
And it might be the case that the post-singularity society can manage a greater growth rate locally in whatever they care about (computational efficiency, say) than they could by the (basically linear or quadratic) growth rate you'd gain from galactic colonization.
Then there is the optimists case.
A sufficiently advanced technology is indistinguishable from nature.
Wrap stars in light-year deep computronium foam, and the surface temperature of the foam is...
Sun outputs e26.5 J/second.
A sphere of radius 2 light years has 50 square light year surface area. The sun emits about 1 degree kelvin over that radius. That is warmer than black body radiation: so a much smaller sphere can be used, which would place such a matryoshka sphere as being in homeostasis with the microwave background radiation. (The real problem is that it might also occlude other stars and thus be noticed)
Move such systems out of the main part of the galaxy (the better to avoid super novas and the like). Wrap starlight around yourself to finish the invisibility job. And now you have invisible, massive civilization that looks a lot like dark matter.
Leave some of the galaxy alone as a preserve.
Noise is waste. Presume an efficient civilization, and you cannot hear them.
Or, transcend. What if it is easier and more efficient to move computronium into more traditional dark matter than it is to visit another star?