Because that was supposed to be a source of humor that was to be mined for the series. Mrs. Roper was perpetually “horny” and Mr. Roper (the late great Norman Fell) was supposed to be irritated and/or lacking the desire to accommodate her. His voyeurism related to watching roommates Chrissie and Janet was supposed to offset this and establish his character as being a “dirty old man.”
Prior to the discovery of Sildenafil (trade name: Viagra) older men were a source of humor as their sexual desire was similar to a dog chasing an automobile: Even if they caught it, they wouldn’t know what to do with it. Additionally, sexual harassment was treated as a joke in the 1970s, and older men were nearly always shown to be the perpetrators. Mr. Roper’s character was supposed to be amusing because he was apparently unable to maintain an erection, and because his voyeurism was harmless and not creepy like it would have been.
As far as Mrs. Roper: While the actor Audra Lindley was an attractive woman for her age, the thought of older women as sexual partners was (and often still is) treated as a joke by lazy comedy writers. So humor (itself, a broad term) was derived from making Helen Roper constantly seek out the sexual companionship of her husband, only to be constantly rebuffed. Since this was the 1970s and daring ideas were only rarely entertained, the fact that several straight, single men lived in her apartment complex (Jack Tripper and Larry for example) and would likely have been more than willing to “accommodate her” was never addressed.
Frankly, the whole concept should have been dropped rather early, if it even needed to be included at all. They weren’t going to cast an unattractive actor for the Mrs. Roper role, she wasn’t slovenly or grossly overweight and she was never portrayed as being unpleasant. The “no sexual desire” dynamic didn’t really work as the writers never provide a solid reasoning for Mr. Roper not to be attracted to his wife. Additionally, the running gag was never going to be easy to sell to middle aged or older audiences, as most men would be more than willing to engage with their life partner (provided that they have the ability to do so) at any age.
It was a rather silly joke that should have been quickly dropped after the first few episodes, if it needed to be in the series at all.
