Searching for ET is quite different from searching for signs of life. Being an ET would imply intelligent, technological life. Thus it would mean searching for communications signals from them. Communications is mainly done using radio wavelengths, at least for humans. Radio is relatively uncommon in the electromagnetic spectrum coming from natural sources like stars, in fact the Earth outshines the entire rest of the solar system, including the Sun in the radio spectrum. There's a few large natural sources of radio waves, such as neutron stars and fast radio bursts (whatever they are coming from). But generally, if you see a lot of radio coming from somewhere, it's a good chance that it's from an intelligence.
But looking for signs of life, intelligent or not, we can use a lot of different ranges of the spectrum, including radio and optical, but also infrared, UV, etc. And in fact we do use all of these ranges. From the surface of Earth, we can use optical and radio, as these sources aren't absorbed by the atmosphere. And from space, the entire spectrum is the limit.