Why can't a Visitor Oyster Card be bought in London? Why does it need to be bought in advance and sent to your home address?

Because it's for visitors.

The point of it is to get more money out of tourists (because the deposit on it isn't refundable) and to give some offers with it. There is no other difference between it and an ordinary Oystercard. To be honest, an ordinary Oystercard is better value because you can return it and get the deposit back at the end of your visit.

Just buy a Regular Oyster Card when you arrive- I have used mine for several years ( just make sure there are some Money left on it) and even let a friend Borrow it - and they had no problems using it

You can use a debit card with contact less instead just as well.