Primarily because they aren't going off in someone's hand or during holstering.
If you drop a duty belt with a gun in a retention holster and it fires, what could have pulled the trigger? Inertia, but no foreign object.
Almost any discharge of a gun securely holstered precludes a foreign object pulling the trigger. Exceptions do occur, primarily with "big mouthed" holsters that allow for a weapon light, which have enough room for a foreign object (say, a handcuff key) to get inside.
Forensics will show if a gun was fired in the holster, and were the muzzle level was when it occurred. Once the muzzle is deep enough, it's not a finger.
At the institutional level, the event will generally be recreated with a non-firing cartridge in the chamber but the gun weighted the same. If the recreation also touches the gun off, that's a pretty solid sign.