First, simple math. $20K > $10K. If you have 100 employees, and they donate $10 and you donate $10..that's $2K. If only you donate $10, that's $1K. In other words, if you are gonna ask this silly question, it should at least be "why not just give $20K per employee".
Secondly, Apple already donates to charities (google it). At what point does a company donate enough that people stop asking "why don't they donate more?" (especially keeping in mind that Apple's priority is to its shareholders).
Thirdly, this is an opportunity for employees to donate money to a cause they believe in and have it essentially doubled. I'm not even sure how else to explain this, except to assume you've never donated money in your life before. For some people, this is a very personal thing (maybe their wife died of cancer)...they want to donate and this is just an little bit extra goodness. Having the company just write a cheque isn't even close to the same thing.
Finally, this makes donating (not just money either, but time and other services) a culture aspect of the company now. The impact of engaging and involving employees can be considerably more significant than just a cash lump sum payment.