One sided hospitality - AIBU?

I have recently befriended 2 other grannies at the toddler group I attend with my 2 yr old grandson. Both seem kind, friendly & I enjoy their company. We have organised outings & had the occasional adult evening too; all good, except for one thing…. They have happily accepted several invitations to play dates & lunch at my house, and we have all had a lovely time. However, we have never been invited back - in fact I haven’t even stepped foot into either of their homes. I’m beginning to feel rather resentful of this and feeling a bit used. Both friends appear better off than me, bigger houses, retired, comfortably off, time on their hands. It’s such a shame, as my little grandson loves his friends coming over to play, but I’m tempted to pull the plug on it, now. Any thoughts / advice much appreciated!