Why is IP67 not an Ex approval?

Because the two markings cover entirely different risks.

IP67 describes the protection of the device against dust and water – in other words, the protection of the robot from the environment.

Explosion protection describes the protection of the environment from the device: it must be ruled out that the robot ignites an explosive atmosphere. Relevant ignition sources are the battery, drives, laser scanners, charging contacts, radio modules and hot surfaces.

Retrofitting is practically impossible. Explosion protection is a design decision, not an equipment option – see guide to explosion protection.