It is most certainly not due to a lack of cast members. Disney is not exacting when casting face characters - it is all about look.
The actors you see performing in the Pirate Tutorial show, however, are cast based on look AND ability. They are Equity performers and are classified in the same group as the musical theatre principles (B&B singers, Nemo, etc.) as well as improv actors (Streetmo, Warehouse, Ad Club, World Showcase, Monsters, Turtle Talk, etc. etc.)
The meet and greet Sparrows are from the character department and are cast primarily on look. So while its very possible you will see a good M&G Jack Sparrow, know that the quality of performance amongst the various performers is probably going to vary. A LOT.