My DC1 is just in reception and so far I haven't helped out. Here's why: I dont have the time, or more to the point, I don't have the time at the points the people who organise the events do have the time.
I have a toddler, so events starting at 6:30pm are a crap time, I need to get her bathed and to bed then. Later starting (7:30pm onwards) I could get someone to sit with her if DH isn't back from work, earlier, (say straight after school), I could bring her along. But most of the PTA work, so they put on things like "film night" (showing a DVD on the big screen in the hall, selling popcorn, drinks and other snacks, charging around £5 per child to get in) starting at 6:30 so they have chance to get back to do it. So the same old faces have to run it and "gosh we work full time and still manage to help" - well done. But the SAHM's who "have lots of time" are more likely to also have a pre-schooler as round here, most mothers return to work once they've got the lot at school. (I have also suggested putting these on earlier would mean that most children wouldn't go home and have tea first, so they could also run a BBQ selling hotdogs and burgers and parents would buy those.) It might look like we have an opportunity to help as if DH is home from work early enough on those film nights, he'll take DC1, but we couldn't commit to it and the last 2 times, I've only told DC1 Daddy would take him to the film once DH had texted me to say which train he was on.
DH also often has to work on Saturdays, and like many DCs in our town, our DC1 has an activity on a saturday morning (we do swimming lessons then, others do rugby or football). I'd find it easier to help on a sunday afternoon, but we never have events on a Sunday.
Committees to discuss planning and events are also held at 6:30 - I have commented when directly asked if I'll come along that sorry, can't do that time, got a toddler to get to bed, yet they never move them to a time that's less suitable for the "same old faces" so they never get new people.
I have baked for cake sales, sent DC1 in with money for various events, got sponsorship for the events they were doing in the day he wanted to take part in, and as this year we'll miss the summer fair again as it falls on a weekend we're probably going to be away, I'll just throw in £20 and be done with it. They never seem to need anyone to do stuff that can be done from home in advance other than baking, so they end up with shedloads of cakes and noone to run the stalls.