[Cryptography] Stupid question on S-boxes

[Henry Baker]

For quite a while, S-boxes have been designed to
resist linear and differential cryptography.

The problem with small S-boxes is that you need
a lot of diffusion to spread the confusion around,
and you need a number of "rounds" to achieve this.

But now that we know a lot more about how to design
S-boxes, how come we don't skip the Feistel stuff
and round iterations entirely, and simply use larger
S-boxes?  I.e., if there are constructions which
build large S-boxes from smaller ones, why don't we
just do that?