lordy, I'm guilty too. DH and I work opposite shifts so if he's here when I am he is normally sleeping off a nightshift and therefore we do other things with the kids as opposed to inviting even MORE round.
on top of that I work long shifts and when I am here I'm catching up with seeing the DC, doing the shopping or general housework etc etc. My kids are also younger than DD1 who is invited on playdates and any we have had haven't been particularly successful as DD2 has felt left out having waited on DD1 to return home only to be 'ignored' if her friend is here too - the eldest 2 share a room and the baby naps in the afternoon as well
Lordy - we're just a shit option for a return playdate - roll on summer holidays and we'll be having to have everyone round as an apology
there are many reasons I never get round to it, being far too busy and not thinking that it was a tally scored activity is one of them