“The problem with that comparison is that when the Spaniards developed the ability to build ships and navigate the seas, they were not the only ones, and they didn’t just do it once and then quit.”
They would have quit if they found it wasn’t worth the time and effort.
“Likewise, if an extra terrestrial civilization had that technology long ago, why didn’t they keep coming in waves like the Spaniards - and if one advanced civilization could do it, why not others?”
There may not be others. I suspect the spontaneous origin of life may be much less probable than things like the Drake equation considers. Especially since such an event requires overcoming the problem of entropy which I think is greatly underestimated by both physicists and biologists. That of course leads to theology regarding a creator and his origins.
There are many flaws in the Drake equation but one of them is the definition of “intelligent life”.
All it would take is one intelligent species to develop Von Neumann probes:
https://en.wikipedia.org/wiki/Self-replicating_spacecraft#Von_Neumann_probes
Such probes could be able to manipulate DNA to create “life”, including “intelligent life” and make it appear anywhere in the universe with even semi-Goldilocks (not too cold, not too hot) environments.
If we don’t blow ourselves up first we will be able to do it.
Then if you add the possibility/imho likelihood of “wormhole/portal” travel throughout the universe—even if at micro levels—the spread of these machines/DNA could be ridiculously fast.