13.17Why omit the 11th on G13?

G13 theoretically includes C.

But G7 contains B.

C against B creates the same 3-versus-11 issue.

So a standard dominant 13 voicing often omits the natural 11.

Thus:

G13 ≠ “all extensions through 13 must literally be present.”

Chord symbols describe harmonic color and available structure, not a mandatory inventory.