This just isn't true. There are current value and belief systems, even ones held by people with a relative large amount of political and economic power, that do in fact recommend such an expansion (someday).
https://d33wubrfki0l68.cloudfront.net/0784d9d63336606d4ca025...
This may be at least in part due to the shifting of energy-intensive industries to countries like China, but it does show that countries can continue to grow their GDP (per capita) while dropping their energy usage per capita, which bodes well for the future.
There are definitely reasons to believe that.
You certainly don't want to be in a galaxy where someone else does it first: if you can't coëxist with them, you're probably going to be wiped out.
Large-scale projects like Dyson spheres would, I imagine, require vast resources taken from multiple solar systems. So even if you're not colonizing every star, you're probably mining from many of them.
Anyway, just a couple of reasons that spring to mind. Given our data set is exactly one intelligent civilization it's hard to draw conclusions, but expansionist pressure seems to be the safe assumption.
Well, since the problem of even figuring out if you are alone is nontrivial why would you opt to do such a astronomical operation because of a hypothetical? We most certainly would not.
Why the pessimism? We do know that we currently do not have any plans for massive expansion through the whole Milky Way, and even if we made such plans, we would definitely include safe zones and ample space for other species we might encounter. If we are that good, maybe other species are no less good? That's not an unreasonable assumption, though ultimately anthropocentric, of course.
But the point I was trying to make was different. I should have said "There is not enough reason". There are just as many hypothetical reasons for as there are against. We really don't know.
>expansionist pressure seems to be the safe assumption.
Then again, the universe is very, very large...
We haven't really done such a great job on or own planet. What makes you think would do any better in the stars?
Possibility (b) seems at least somewhat unlikely, given that we (at least) evolved over millions of years to the very particular conditions of Earth 0. So finding somewhere 'better' would likely involve some gene-gineering to adapt, and maybe involve a shallower gravity well than we currently inhabit.
Possibility (a) is a really good reason to spread out a bit, but you likely don't need more than a few insurance policies... Certainly there's no need to aggressively conquer the entire galaxy: Given difficulties of communication and transport over long distances, there's likely some real question of what 'conquer' would even mean...
It occurs to me that it would be interesting to look into how much stars drift apart from one another over the course of time involved in getting from one to another. Consider that the launch window for mars comes up once every two years; what are the launch windows like for stars, where travel might take hundreds or thousands of years? Maybe it takes a thousand years to get a scouting run done, and then it's infeasible to follow up with an actual colony ship...
- Cyclical cultures, filters, and large time frames are not properly taken into account. If you take them into account, colonization of the whole Milky Way seems unlikely.
- Life could occur only very rarely.
So basically, the missing premise is that if all of your conditions are fulfilled, then a civilization will colonize the Milky Way and has done so far enough in the past that we can easily observe it. There is no reason to think that.
Now here is the bad news: Even if we accept your original line of thought, that would still be entirely compatible with the view that some intelligent species is just about to colonize the Milky Way and just in front of our doorstep...
I'm just kidding, of course. Like others have pointed out, this "research" is almost entirely based on speculation. We don't even know how likely life would arise if all conditions for life are fulfilled, let alone whether that also means that intelligent life will arise or with which likelihood. We know nothing.
The only thing that we can say with relative certainty according to our current knowledge is that if intelligent life develops with high probability whenever the conditions for life are met, then it is a bit harder to detect then we thought or Xenoarchaeology will be booming some day in the future.