I have a circle of school mum friends. We all live close to one another. One of the 5 of them does invite me in for a cup of tea after school if it's just me and her with our respective kids.
The rest almost NEVER invite me to their house. They are more than happy to come to me - and do virtually every week (which means me making tea, getting drinks/snacks for kids) for 6 adults and 12 kids - as inviting one tends to end up being an inviting them all thing. And while I do genuinely enjoy having company, it gets pretty exhausting always being the one to be the host, particularly as it means a monstrous mess after they leave, which I have to clear up.
Now admittedly we have the biggest garden, but they all have gardens of a pretty reasonable size. I get the impression that none of them want the hassle of cleaning up or catering (by that I mean making tea and providing some biscuits/cake) so they just don't bother to offer.
Similarly, we have regularly (several times a month in summer) have them all around to ours for impromptu BBQs or break up day parties or dinner or get togethers after village events. Beside being invited to a Christmas drinks party at one of their houses once, we've never been invited to any of their houses for anything similar. In fact I've only ever been to their houses for the occasional playdate and our girly get together nights.
I think they genuinely all enjoy the get togethers. But I find it seriously rude/upsetting that none of them ever think of returning the favour.
If I just stopped inviting them, I do wonder whether any of them would step up and offer but I doubt it. We'd just not see them. Which would be a shame, so I keep doing it. But am getting fairly annoyed now.