Why do we need to expand? Our population growth is already levelling off.
Why do we need to send _our bodies_ rather than with AI, or with our consciouses uploaded into computers? (a la Charles Stross' Accelerando or Greg Egan's Diaspora)
If we're sending AI probes around, why do we need to take all resources to the extent that we prevent other species from developing?
Most of the fundamental science that we want to do can be done right here in our own solar system. There's just not a need to expand.
Yes, of course, they will be AIs or uploads or body printers. Even for an advanced civilization, shipping bags of meat to other star systems would be wasteful. That doesn't mean they won't want to use resources, or that there would be no evidence of them if they were already here.
edit: There's certainly scenarios in which the first species to explore a galaxy doesn't literally prevent other species from evolving. But that doesn't mean they wouldn't be obviously detectable. The likely number of starfaring species in the galaxy given our observations is zero.
Like, the range quoted above is from "10,000 to a few million," so a factor of 200ish from high to low range.
If you think that there are 10, per galaxy, why don't you believe instead that there are 0.3 per galaxy (only off by a factor of 30?)? How much confidence can you have in your highly speculative variables that your output can be constrained down to just a single order of magnitude or so?