TOYOTA HILUX 2KD-FTV ENGINE GASKET

The Toyota Hilux 2KD-FTV engine full set gasket is one of those gasket set which come without the cylinder head gasket in it, I'm still thinking why they want to sell a overhaul set gasket without the cylinder head gasket, is it because the original cylinder head gasket is durable until the overhaul it still no need to change, or the cylinder head gasket actually have the 5 range of thickness so they want the owner to choose which thickness they want when they buy the set? Or it had other reason which I would like to have your opinion.

For the other type of engine that the Full Set Gasket didn't included the cylinder head gasket is like 14B-T, 15B-FT, 1KD, 1KZ-TE, 2L-T, 3L and 5L.Other than Toyota, the Nissan also have some model apply this concept like their engine of CD17, CD20, LD20T, LD28, QD32, RD28, TD23, TD25, TD27, TD42-T, YD25DDT and ZD30DDTI. Isuzu also have some engine range apply this concept such as 1988-on 4BE1, 4EC1, 4FG1 and 4HF1 (I think all the 4Hxx series also apply).