Because Di2 are mostly extremely reliable too.
Mechanical gears will also break if knocked or crashed, cables can break or get kinked, levers can be smashed… 🤷🏻♂️
When have you ever managed to kink or break a gear cable during a ride? The only time I’ve ever managed it has been while performing cack-handed maintenance.
The derailleur can get bent. The lever can break. The pivots can start sticking. These are all things that can happen with Di2.
In addition, it can randomly stop working for no apparent reason.