Why do we keep designing new static s boxes?
There are simply a long list of properties for S-boxes that are relevant, see for example these questions:


Desirable S-box properties
S-box design criteria and random sboxes


In addition people have been trying to design S-boxes that are fast to implement in hardware or in software or which can be masked efficiently. For example the Midori S-box is designed to be especially energy efficient.

The search space for S-boxes is actually quite big as well; for 4-bit S-boxes (pretty small) there are $16! \approx 2^{44}$ possibilities.

This might be a nice read: Cryptographic Analysis of All 4×4-Bit S-Boxes