Why is the Nvidia 9600M GT not detected on my mb pro?

The version of MacOS likely is an issue on where you see it and don’t. The About This Mac in some versions won’t show the dedicated GPU only the Intel intergraded graphics engine. Going to the System report will show both under the Graphics/Displays heading as an example in my 2012 A1286 15” MacBook Pro:

Now some models did not offer a dedicated GPU option so you’ll only see the intergraded graphics engine within the Intel CPU. As you didn’t tell us your exact systems I can only surmise you have one of these. A good way to see what the systems specs is to plug in your systems S/N here EveryMac - Lookup to double check.

If you still see something odd take a screenshot of the listing like I did so we can see what you see Aggiungere immagini ad una domanda