Perhaps it is because astronomers trained their telescopes at Venus, a planet about the same size as Earth and one that has a very definite greenhouse effect, and the sight made an emotional impression that overbalanced the reason.
Sure, if you have methane and CO2 at the kind of concentrations Venus has you do get a greenhouse effect, but Venus is NOT the same as Earth. The concentrations are extremely high, and there are no correcting mechanisms (like life) to reduce methane and CO2.
Venus Temperature Still Uniformly High
This recent high temperature formation of Venus is consistent with its high surface temperature, > 462 °C (736 K / 864 °F) measured by Venera and Pioneer Venus (PV) probes, identical in completely different regions of the globe. The surface rocks are hot enough to melt lead and zinc. Proof that the temperature is driven by interior heat comes from the striking uniformity of this temperature - only one or two degrees deviation from pole to pole and on the day and night sides. Given the Venus day/night cycle of 117 earth days, it is obvious that incident sunlight, which is completely extinguished in the lower cloud layer, has absolutely nothing to do with the surface temperature. The global lower cloud layer, at 48 km over the entire planet, is also driven by the great interior heat...
http://www.firmament-chaos.com/planets_venus.html
Venus' atmosphere is 96% CO2, while Earth has 0.038% CO2. It's a big stretch to think that Earth's CO2 would generate a significant greenhouse effect.