Perhaps one of the reasons is the requirement now for form filling, 2 references, a basic DBS check often (18£) or enhanced DBS for working with children or vulnerable adults even if its only for serving teas.
When my children were younger, school was regularly asking for help hearing children read, playground duties, helping children dress/undress for gym, swimming etc., School governors, PTA, accompanying class on trips, etc.
Volunteering in a charity shop, for eg, meant filling in a form with name address, phone number, now its a two page form, with all manner of impertinent questions.
I volunteered for a few weeks in the next town, paying £1 to park, and petrol, fine if I was enjoying it and feeling helpful but I wasn’t. So I left.