Of course that is not a property of RAID-1 but only of a dumb implementation of RAID-1. There is no reason whatsoever why a request to read 128MB of data could not be satisfied by reading 64MB from one and 64MB from the other drive. After all, both drives have the same data and can operate in parallel.
That the developers do not want to do it does not mean it can't be done, and indeed it is done by certain hardware-based raid controllers.