A few different reasons:

1. People have been reporting issues about "frozen" compiles, when in fact it's just very large dependencies that take a long time to build. This output will make it clear that work is happening, just slowly.
2. I've had CI timeouts in the past due to lack of output for 10 minutes. Basically, same as 1, but impatient machines instead of impatient humans
3. It can be informative to see the output as it happens

Downsides:

1. Subjectively more confusing
2. Arguably people don't want to see the individual modules compiling for each dependency

CC @nh2