Lower block times are clearly desirable. Why does not Bitcoin reduce block creation times to 15 seconds just like Ethereum in this case? Because it would lead to too many forks. The main problem with forks is not the lack of rewards for stale blocks. It is the convergence time and wasted computation.
Let us suppose Bitcoin will reduce block creation times to 15 seconds and call it Bitcoin-15 for the sake of the discussion:
Is there a reason Ethereum does not have as many forks as Bitcoin-15 would? Or perhaps it does but the forks are not as harmful? For example, if the transactions from the uncle blocks had been merged with the transactions in the main branch, the computation would not have been wasted. However, the transactions from the uncle blocks are not merged. The fact that the miners of uncle blocks are rewarded does not solve the problem of wasted computation or convergence time.