You can build better receivers but eventually only so many photons are being sent. Prior to 1921 people were sending out very little radio waves and the signals where vary weak. The you might move things back to say 1920 with the very first radio station but it really wasn’t putting out much power. Things ramped up fairly quickly though, at least assuming something built a truly massive detector at 100+light years.
For comparison look at what we need to do to detect messages from the voyager 1 at 1/400th a light year, realize that’s sent from a directional antenna and we know exactly where to look for it. A signal that’s ~1/1,600,000,000th as strong would take some serious hardware even with more advanced technology because you can only collect 100% of the photons in a radio transmission quickly you need a physically larger device.
For comparison look at what we need to do to detect messages from the voyager 1 at 1/400th a light year, realize that’s sent from a directional antenna and we know exactly where to look for it. A signal that’s ~1/1,600,000,000th as strong would take some serious hardware even with more advanced technology because you can only collect 100% of the photons in a radio transmission quickly you need a physically larger device.