Donny one of the points I was trying to make in an earlier post..

An accomplished player that plays rhythmic style in the left hand with moving bass notes and chord structure in the right hand that goes beyond a basic 1-2-5 .. A player with a much higher skill level, and to be fluent as a singer is very rare ( sure Barry Manilow)..

The arranger keyboards cover up the ability to excel as a player, but when you have relied as an instrumentalist and developed your skill, it is much harder to sing or even want to sing. Nor is it a necessity.

Just as an example, comfort zone counts... You prefer to play in the key of "C", Play your material in C# or B, does it effect your vocal?

It surly will, because you are not in your comfort zone, and the added awareness and thinking about the different key.

Now for let's say a accomplished piano player that takes much more skill, the concentration to add vocals is much harder to obtain..

Not an excuse but a fact..