With crowded AM channels, going non-directional means very low power... often well below 1000 watts.In the case of AM radio stations, the added cost of maintaining and operating the transmitting hardware to provide a directional signal could be eliminated by using a low power omni signal at night (maybe up the power some for daytime, still omni) and promote streaming the AM radio station.
Major market radio stations don't consider electric bills to be a significant expense. Those stations that do reduce power are doing so because they lost their transmitter site or the site became so valuable they literally could not keep from selling it and the rebuilding cost of land, engineering and getting permits was not practical.
Many if not most cases where a station does cut power occur where the station has a translator and the AM is only needed to keep the translator operating and, otherwise, the AM is valueless in covering the market. In those cases, the AM is only operated to justify the translator.
But to save on the light bill is not the objective in nearly all of these cases.