We’re all different, and all have different needs. Some people, upon retirement, feel they no longer want to commit themselves to set routines which volunteering would require them to do. Others have other commitments which don’t allow them the time to give to a regular volunteering role.
It seems, too, from what others have posted, that there are many hoops to jump through these days in order to volunteer including detailed form filling, on line courses and interviews, and just don’t want to do that.
Of course, for some, volunteering is a blessing as they may find the sudden change from the ordered structure/routine of work to a less ordered routine of retirement difficult to deal with, and feel at a loss or even bored without that structure to part of their week.
However, although it’s a noble thing to do, volunteering isn’t for everyone.