People weren't that bullish on the conclusiveness of those results at the time. (Except for the radio signals.)
Even though we know that our atmosphere is maintained by life, nobody did really know if something else could create oxygen and methane. And betting on a specific light absorption spectrum is quite a hard thing to justify.
Today we know that those things are rare. And they are orders of magnitude more intense than the unexplained phenomenon we see on other planets. But at the time, we didn't have proof.
And the radio signals are a pretty unhelpful indicator because you could have looked at earth with life on it at any point within the last three billion years and there have only been radio emissions for the last 30-millionth of that time. And they might become pretty rare or completely gone in another 30-millionth.
For mammals, the radio-emitting time is still only a 2-millionth of the total.
The thing about radio-waves is inverse-square-law. I don’t understand why we’d expect our nondirectional radio/TV broadcasts to be distinguishable from background radiation once you get past, say, a light year or two from our solar system.
Earth SETI is basically just looking for directed signals: hypothesised to be either intentional communication attempts directed at us, or signals that we happen to be in line-of-sight of. Omniidirectional signals that could be detectable over interstellar distances would require immense amounts of power at the transmission end, and anyway may even be detected by conventional radio astronomy observations.
The popular conception is that our undirected, historical tv/radio signals might be detected by ETIs - and that therefore this is what our SETI efforts are looking for. In practice, the former is unlikely using known scientific principles, and the latter isn't really being attempted for the same reason.
I don't have a reference for you, but I recall a study a decade or two ago that suggested that we would only be able to detect our strongest radio signals out to a distance of about 10 light years with existing telescopes (I may have that number wrong, but it was not large). That said, with new telescopes, particularly things like the Square Kilometre Array, we have higher sensitivity, and more importantly, we have the angular resolution to measure whether an excess of radio emission is coming from a star or from a planet around that star. So, even a weak signal can be detected, and if we were to find such a signal originating from a planet rather than another object, we could make a pretty good guess that it is artificial in origin.
I dunno, one of the slam dunks was the spectrum cliff showing vegetation. What if the energy production system on Planet X wasn't based on Mg-bearing chlorophyll? And plants have only been around for 1/7th to 1/4 of the planet's history, so it's a crap shoot any way you slice it.
Even though we know that our atmosphere is maintained by life, nobody did really know if something else could create oxygen and methane. And betting on a specific light absorption spectrum is quite a hard thing to justify.
Today we know that those things are rare. And they are orders of magnitude more intense than the unexplained phenomenon we see on other planets. But at the time, we didn't have proof.