The short answer appears to be that in workout mode, so that you can focus on hitting your power/cadence numbers without having to account for the additional variable of a changing slope, Zwift artificially “flattens” any course you happen to be on–i.e. even though a course may have significant inclined/declines, and even though you can see the % of the slope of screen, you’re essentially riding a flat road. As a result, your Kickr doesn’t tell the Climb to do anything.
See here, if you want more conversation about it.