For those with less context into social dancing, this is because the typical default "stance" is offset to avoid stepping on each other's feet - each partner's right foot is roughly aligned with their partner's midline.
I presume that is in ballroom.
For Latin, Tango and Swing which are all mentioned: Simply not true. Your hips are so far apart that it doesn't matter.
That being said: you ideally have a key/change pocket in your pants. Or even better, a beltline interior pocket. The latter is where I keep my keys when dancing tango. On the right side btw. You can't tell from the outside.
These pockets are sadly not available in off-the-shelf clothing any more since maybe around the 1940's.
The reason is twofold: you won't feel your keys/change as much and from the outside you wont have anything ruining your leg's silhouette. Left or right. :-P
Offset stance, and closed position, is used in many swing dances, for example Lindy Hop and Balboa, so your statement is simply not true ;)
Many Swing dances (and including the most popular one, Lindy Hop) have a closed position.
In Balboa you're generally so close that there is contact at least from the side of the hips to the side of the chest, in more advanced levels often extended up to the cheeks.
Also depends on what "Swing" you're referring to, but even in Lindy Hop I would say I my right hip is close to my partner frequently enough that this would be an issue, in partnered Charleston and various Shags there is regular side-by-side connection (connected on the right side for the lead), and Balboa is danced in an offset close embrace position where your hips, while not necessarily in direct contact, will often brush against each other simply due to proximity.
It absolutely can be true for Argentine Tango, depending on the style of embrace.