Let's say there's somebody out there 100 light-years away. A broadcast they make registers as a million watts one hundred kilometers away (that's an absurdly powerful artificial signal). By the time it gets here, if I got my calculations right, the wattage at our end would be about 0.000000000000000112 watts, considering a perfect vacuum and no interference (a star between us). And we're supposed to detect that amid all the other electromagnetic radiation from closer and stronger natural sources? SETI is a fun exercise, but it's a shot in the dark.
I sincerely appreciate the lesson in radio astronomy. Thanks for helping me understand the situation better.