You aren't missing anything.
It's a feature that Multiple Hash doesn't have, as I hadn't encountered that particular use case before.
You can use the Advanced Editor to select all the input columns quickly (Right click on the Multiple Hash, and select Advanced Editor. Swtich to the Input Columns tab, and then click on the check box on the left of the column header "Name".)
This will halve the clicking that you have to do.
Unfortunately, there isn't yet an equivalent check box for the output columns.
I'll move this to a work item and see what I can do to add the output column ability.