1. They need energy and any suitable source will do
2. Space is mostly the same. Why explore another star's gas giant when they have their own?
3. They just like hanging out in their Matrix...which is what they would be doing if they traveled twenty light years
So why go to the trouble of changing your location?
The whole notion of "expansion" is such a weird 19th century concept that just doesn't seem to go away. Space isn't the Earth, other star systems aren't unexplored continents or islands in the ocean - they're islands in spacetime.
I've yet to be presented with a rational explanation as to how any civilisation would be able to a) stay stable in terms of biology, culture and technology over geological time scales and b) have any means of expanding itself over distances that make communication practically useless (e.g. even at "just" 25 light year distance, two-way communication would take 50 years) and c) are somehow advanced enough for interstellar travel and presumably efficient terraforming or genetic modification yet unable to control population growth and resource utilisation...
Aside from that, almost all papers on the topic including this one focus on subluminal space travel.
Sure, once you incorporate technology that goes beyond your current understanding of the universe, many counterpoints go out the window, but this level of technology only serves as an argument against the need for expansion in the first place (with the exception of "because they can").
FTL would be fantastic for research and science expeditions, but expansion into random nearby star systems would be even more pointless, since such civilisation would be able to pick exact/close to exact matches to their home world as candidates for settlement. Such worlds could then be thousands of lightyears apart, as neither communication nor travel would pose a problem. This would also easily explain the lack of (radio-) signals, as exchanging information would be carried out via FTL travel, not radio waves. Why would a civilisation capable of FTL choose to go through the trouble of terraforming or building megastructures around random stars? Wouldn't they rather choose to harness the power of black holes then? A planetary mass BH could power a civilisation for trillions of years for example.