I think it's because of the ask of volunteers is rightly bigger these days
I volunteer in an animal rescue type environment
Historically volunteers will just wander in and out, and people somewhat expect to do that still. We frequently turn away half hearted volunteer attempts and people get really angry about it
For example people don't want to commit to regular days (which is understandable in a climate inwhich people work) but we need that in order to invest in training someone up. It's incredibly hard if we have no idea how many people might volunteer in a given week, we can't then figure out staffing, many places might need dbs, you to do mandatory training eg safeguarding. We can't invest that in someone that maybe comes once a month, maybe.
The people with the most time these days are kids, which just aren't appropriate in a lot of roles.
Lots of people expect that their 14 year old can come to a kennel on a random Tuesday and walk dogs. Most of our dogs aren't appropriate for a 14 year old even if the insurance wasn't insane
We've had people turn up to volunteer with no prior arrangement and bringing their kids with them.
Theres also a better understanding these days that sometimes paid staff, or consistent staff are better. For example people might randomly volunteer to teach in Africa, but now it's better understood that its disruptive to kids learning to have untrained people who possibly don't speak the language come for a week and it would be more effective to pay an actual teacher rather than someone fundraising to fly themselves out. Gone are the days where you could wander into an orphanage to volunteer like that
In my side of things (wildlife) there is a lot of skill involved and frankly people have to invest a lot of time to get there. For example hand rearing birds, the syringe feeding is hard and delicate, you can't speak to them etc. So most of our volunteers that are only coming once a month end up doing unskilled work, that frankly is less rewarding.
People tend to want to spend time with the animals, but really it's stuff like cleaning out, admin and washing up that need doing, but people don't want to do that.
My mum used to volunteer with wildlife, and she was allowed with no training to take stuff home to handrear in a completely inappropriate environment. She also wasn't working at the time, so she volunteered at a school with no dbs etc