However, first, Fermi's observation was that the age of the galaxy was such that it would be possible for a technological society to colonize the entire galaxy. So where are they?
The answer that makes the most sense is simply that distances between technological societies are vast and interstellar travel is extraordinarily expensive and time-consuming. It would be a one way trip. Tech societies are likely stranded in their stellar systems. There are likely no warp drives to help here, or there.
Mathematician Ian Stewart has an interesting and amusing answer. All tech societies will eventually think of the Fermi paradox. Maybe what's happening is that they are all just waiting around for contact, one which will never come because they're all sitting around waiting and not venturing out. On SETI, maybe they're all listening and nobody's actually transmitting.
Myself, I think the galaxy is full of life with many tech societies. The numbers are just too large for us to be alone. Whether we will ever be contacted, I don't know. Somehow, I think it's unlikely. The distances are just too damned huge.
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