Revealing mistake: At the end of Lucy and Van Johnson's dance, Ricky and everyone else clap as Lucy walks through the curtain to head "backstage', and in the next shot just as Lucy walks through the curtain we catch a glimpse of Desi Arnaz standing there waiting for his cue. Then in a few moments Ricky, Ethel, Fred, and Caroline all rush in excitedly to congratulate Lucy. (00:24:55)
Other mistake: Lucy walks out on the balcony and sees Van Johnson sleeping on a lounge chair by the swimming pool. The photographic backdrop shows automobiles, trees, mountains, the Roosevelt hotel and many other buildings of Hollywood but when Lucy goes to the pool and sits on the lounge chair next to Van Johnson, the background is just a plain painted backdrop with no cars, trees, mountains or buildings. This plain backdrop also appears in the episode with Harpo Marx.
Answer: There is no reason. It's a just a silly plot device, typical of the era. Women characters were often portrayed as making uninformed assumptions or decisions.
raywest ★