Bitcoin Wallet has been deterministic for a long time. In fact, it was among the first to switch to HD.

The main reason the seed is not exposed as 12 words is because in many cases they are not enough to back up your whole wallet. A wallet can consist of more than one seed (if one was detected as unsecure for example), plus a number of random keys from the old days. Just writing down the current seed will make the user loose the older ones – but due to address reuse people still do receive coins on old seeds and keys.

And the other reason is no one has implemented it. Bitcoin Wallet is open source and free software. Anyone is welcome to add features themselves and offer it as a pull request.