Is "couples" the reason? I (implicitly, no idea why now that you bring it up) read the other descriptions of weeklies as not-with-partner events. Perhaps not strictly not-with-partner, but default without.
"Couples" puts everybody in involuntary performance mode. It starts super harmless, the "and" element (the side in a couple that came along) having a natural desire to be good with the friends of their partner (the "and" side is prone to greatly overestimate the depth of the relationship that lead to the invite!), the other side (the one that had the "link") trying to make their partner feel good with that group. So far so good, everybody does the reasonable thing, sounds super nice. But that very same thing happens within every couple, nobody in the room who isn't in "prove myself to my partner" mode. Before you know it escalates into a take-no-prisoners battle of showing off which couple functions best. It's certainly possible with couples despite all this, but there is infinitely more that could go wrong along the way.
Yeah, maybe. I never discovered the secret of sending invitations that radiated "please don't bring your partner." Brunch is, after all, an activity for mostly women and couples.
Yeah, girl's night/guy's night does come with a certain expectation, or a stigma even. And is not a brunch. In Germany we actually have an established brunch time event class that literally translates to "morning pint" and, well, your imaginations are probably spot on. The safe path to not couple-centric is usually some form of hobby which tend to lean of gender or the other. And those who defy the gender stereotype will most likely be sufficiently deep into whatever it is to not fall into couple validation mode. But hobby-type pastimes that are also inviting to the noninitiated are indeed rare.
