Oh god - I am probably being talked about as a non-reciprocating mother
In my case there are loads of reasons - some more valid than others:
DH works from home. Our kids know to keep the noise to reasonable levels, or play outside - others don't, and I get fed up having to 'police' the situation.
I genuinely mean to invite people back (perhaps at the weekend) but just never get around to it.
And once you've had someone round then your DC are invited back again, and then you have all the guilt of reciprocating all over again .
There are actually some of DS2's classmates that I don't like - they are rude, badly behaved and break things! I try to avoid DS going to their houses in this case.
If it's really bothering you, then just stop inviting them - no big deal. I think some people are just more 'into' playdates than others. Some mums seem to see it as their 'career' to organise their child's social life, but for me it's just another chore on a very long list.
My kids see their school friends at school, and at home they play with friends in the street (and it's a lot less formal - sometimes they come in for an hour to watch a DVD or something).