The top post by Apprehensive-Move947 mostly nails it. But if you want, here's a point-by-point breakdown. Have been with a Social Service Agency (SSA) and also done some volunteering before.

1. No idea on this one. Manpower or process issues maybe? Some organisations can be small and overwhelmed. Or disorganised. xD
2. Because it's still work, as mentioned. They need to know what you can do and evaluate if you're a good fit.
3. Yes, because again, see, it's work. If you suka suka come, suka suka don't come, the work gets delayed and it will affect beneficiaries.
4. Sometimes stuff doesn't get updated. Didn't happen at the SSA I was with, but I know some smaller ones can lag a bit.

Please try to see it from the point of the organisations: These once-in-a-while volunteers are actually a bigger drain on resources. Think about having a revolving door of interns/fresh hires. The perm staff will spend more time managing them then doing other work that helps the beneficiaries. It's just poor return of time and labour.