Because it is the legal requirement and the 120 metres are defined as such by the FOCA.
The 120 metres apply from the take-off point. So if I am on a mountain, the height applies from the take-off point on the mountain.
That's why I personally don't give any instructions on how to get round this;
If you want to fly above 120 metres, you are breaking the law and if you are caught, you can be fined several thousand francs. You always have to fly by sight and when the small drone is that high you can hardly see it.
There was once a case where the drone pilot flew very far away and published the video; he was then able to prove that he was not flying by sight and, as far as I know, the fine was over CHF 4000. Unfortunately, I don't know whether he was then allowed to continue flying a drone.

The altitude limitation can be removed by changing the drone from C0 to C1 certification. This is very easy to do via the DJI online portal, but the relevant rules must be observed when flying. After approval, you can fly up to an altitude of 500 metres from the starting point.

120 metres is currently the highest that DJI allows for the Mini4 Pro, and that's a good thing.

If so, how?