This has been asked for several times in the past.
I agree that it would be beneficial BTW.
Some of the responses have been that WS do not require anything to set, so the synchronized start for WS is easy and logical to program. However, Manus require items to set (and in different amounts based on different structures being at different levels).
However, I still think this would be relatively easy to account for and code around. If the total requirements are not available, for whatever reason, the box could be unavailable, and/or an error message could pop up indicating such. The unavailability feature already exists on an individual basis for each Manu, for each product, since those options grey out if the requirements are not present.