@woks Because if everything were perfect (no secrets to hide, no conflicts, etc.), it'd just be them going on dates and hanging out. That's swell albeit boring; seeing two people love each other without any sort of dramatic tension are either riddled with sexual tension and becomes porn without the sex or a prolonged exhibition of an ideal pairing of the two characters (and by extension the author feeling themselves for writing a cute couple).
Is it riveting to see two people get along well while actively being recognized as an actual couple by their surroundings? It tugs on our heartstrings for a small bit, but ultimately it's a very short and quick payoff. These instances don't last especially long because they usually fail to hold a majority audience for more than at best three chapters, which is why authors either do stupid plot twists which endanger the couple's bond that piss everybody off ala the many of the series
@Yautja mentions (Otoyomegatari's couple has the main character leaving to become a more suitable partner for his wife, but that pesky Russian invasion begs to differ; Ojojojo has its main male character mostly aloof from his girlfriend due to his sinister past; Tzurezure Children outright ended just as most of the couples became consolidated), or the author cuts the series right where things are at their juiciest to end on a high note (such as Ookami Shounen wa Kyou mo Uso o Kasaneru, which has the main couple exist for merely one chapter before stopping the series). Most exceptions that to this (Bonnouji comes to mind) usually peter off into a droll trot that's pleasant but just milquetoast in terms of interesting plotlines until they too just end. Horimiya and Wotakoi are exceptions to both of these statements, but the former is pretty much coasting on trying to keep interest by focusing on side couples since the main couple are pretty much at the cusp of the peak of their relationship; they can't escalate much higher without having drastic consequences to the status quo (which the author seems to not want to end just yet). The latter, however, is one I'll concede to; Wotakoi's gimmick that the couples are ironing themselves out from their hobbies and their relationship are very capable of keeping a tight hold on its audience. For the time being at least, I'm keeping my eye on it.
I'm all for seeing cute stupid stuff between a cute couple, but I also understand that it gets very boring very quickly and by extension aren't usually a vehicle to tell a compelling story (or are victims to the axe like Magico was, that was a tragedy). Maybe one of these days I'll check out that Mork and Mindy that
@FredFriendly mentions, though.