Broken down rides can't be worked on...Why???

Many times on RCT, I've attempted to work on a
coaster, such as adding an extra train, adding/
removing brakes, elements, etc. after it has
broken down, and before being fixed by a mechanic,
and a red box comes up on the screen saying "Ride
cannot be worked on...Has broken down and requires
fixing". Why is that??? I feel that you should be
able to work on rides regardless of their status,
since they automatically close when they break
down.