Hi Squeak
I see what you are getting at, but you have forgotten one very important point, and that is the boards you refer to are hardware based and so suffer from the limitations that this technology brings, particularly with memory.
As an example
A manufacture purchases a consignment of 100Mb ROM Chips, (These are where the OS and sounds are programmed in) and to keep costs down they will be used in both Workstations and Arranger keyboards.
The design and marketing team decide that for the Workstation 50Mb will be used for sounds, 30Mb will be used for Mega voices, 15Mb for high end Drum kits and 5Mb for the OS, the Rom chip is now full and nothing more can be added to it. (Additional memory can be added for user sounds, if the customer decides to purchase it)
The design and marketing team decide that for the Arranger 50Mb will be used for sounds, 30Mb for Mega voices, 15Mb for SA voices and 5Mb for the OS, the Rom chip is now full and nothing more can be added. (As with the Workstation additional memory can be added for User sounds at the customer’s expense)
As you can see from the above it is not possible to fit everything onto one chip, and so they have to choose the items they think will sell to a particular market, if they did increase the capacity of the Rom chip, this would incur greater expense, which would then have to be passed on to the customer. (Rom chips are a lot more expensive then Ram chips)
Computer based systems don’t suffer from this problem as all the info is stored on the hard drive, and loaded into Ram at start-up. (Ram is cheap so if they need to fit more, the actual extra cost to the customer will also be small)
Hope this helps.

Bill