I believe blocks are spread in time to ensure that any messages will propagate across the entire network in the duration of a block, 10 minutes being a rather pessimistic view of how long it could take for a message to propagate.

The thing is, faster blocks would take less effort to generate, so to get the same level of security as you currently get by waiting 30 mins for three blocks, you'd have to wait 30 mins for 15 blocks. So, nothing would go faster, you'd just be consuming more traffic spamming the network with more blocks.