It is a separate command so that you are not accidentally deleting your footage.
The default keyboard shortcuts for Mac are:
clear in: opt + I
clear out: opt + O
Clear in and out: Opt + X
The default keyboard shortcuts for PC are:
clear in: Ctrl + Shift + I
clear out: Ctrl + Shift + O
Clear in and out: Ctrl + Shift + X
Hope this helps 🙂