What if interstellar travel is just really hard (= no advanced life form has figured it out, and it might just be impossible/not economical), and whatever signals their civilizations generate just aren't strong enough to be picked up by e.g. SETI?

It wouldn't spell "doom" unless you count the sun running out of juice/going supernova. The universe might be packed with life, each such patch effectively isolated to it's own little corner.