Because that was supposed to be the source of humor for the series. Mrs. Roper was always “horny” and Mr. Roper (the late, great Norman Fell) was supposed to be annoyed and/or unwilling to please her. His voyeurism, which involved spying on his roommates Chrissie and Janet, was supposed to compensate for this and establish his character as a “dirty old man”.
Before the discovery of Sildenafil (trade name: Viagra), older men were the source of humor because their sexual desires were like a dog chasing a car: even if they caught it, they didn’t know what to do with it. Additionally, sexual harassment was considered a running gag in the 1970s and older men were almost always seen as the perpetrators. Mr. Roper’s character was supposed to be funny because he clearly couldn’t maintain an erection and because his voyeurism was harmless and not as creepy as it should have been.
As for Mrs. Roper: While actress Audra Lindley was an attractive woman for her age, the idea of older women as sexual partners was (and often still is) considered a running gag by lazy comedy writers. So the humor (itself, a broad term) comes from having Helen Roper constantly seek out her husband’s sexual companionship, only to be repeatedly rejected. Since this was the 1970s and risqué ideas were rarely accepted, the fact that there were several straight, single men living in her apartment complex (Jack Tripper and Larry, for example) who would likely be more than willing to “go along with her” is never mentioned.
Honestly, the whole concept should have been dropped pretty early, if it even needed to be included. They wouldn’t have cast an unattractive actress as Mrs. Roper, she’s not sloppy or overweight, and she’s never portrayed as annoying. The “no sex drive” dynamic really doesn’t work because the writers never give Mr. Roper a solid reason not to be attracted to his wife. Plus, this running gag would never be an easy sell to a middle-aged or older audience, since most men would be willing to stick with their partners (provided they have the ability to do so) at any age.
This is a pretty dumb joke and should have been dropped after the first few episodes, if it really needed to be in the series.
