The big issue here is different understanding of "reasonable time". The galaxy is very large, and the time spans involved are very large, but they are not in th

The big issue here is different understanding of "reasonable time". The galaxy is very large, and the time spans involved are very large, but they are not in the same ballpark of "large", the time gaps involved are so much larger. If one civilization gets their evolution of intelligent life just 1% quicker than us, that's a head start of 50 million years. On that scale, "reasonable speed" required to fill the galaxy (to use round numbers, 10000 "hops" of 10 ly each for 100000ly distance across the whole galaxy) means something like a planet launching a single colony ship to a nearby star once in 5000 years (and the new colony populating the planet and launching another ship in 5000 years, which seems quite reasonable). You don't need FTL, you don't need fast ships, you don't need many ships, just one generation ship in a few millenia moving at 0.01c or even less is amply sufficient to colonize all of the galaxy many times over in that time range. And if someone was 2% or 3% faster than us, then they'd have even more excessive time to swarm the galaxy even if they're doing it at snail's speed. So to explain the fact that the aliens aren't swarming everywhere because they can't afford it means accepting that either they launching a single generation-ship every 5000 years is a huge expenditure (which IMHO it wouldn't be even with our current economy size, definitely not an issue with the tech of 2100 or 2200 assuming at least some growth because at that point a Musk-type figure with a wish to "make humanity interstellar" would be able to afford it from private funds) or that we're the first (only?) life in galaxy or that all the aliens before us didn't try to expand, like, at all - because getting just one single rich eccentric every 5000 years (like a bunch of billionaires here are) would be sufficient to make the galaxy swarm with juuust a bit higher tech level than ours now.