system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is Jailbreaking an iPhone so difficult?

Couldn't a tethered device like a computer just erase everything on your phone's boot drive and then re-add the software with the tweaks? Why can't developers build their own operating systems and add them to an iPhone once everything is erased?
gpt
On any computer, whether that be a mobile device like an iPhone or a traditional desktop computer, you can only easily write to the hard drive by using the operating system - by telling the operating system what to write onto the hard drive.

> Couldn't a tethered device like a computer just erase everything on your phone's boot drive and then re-add the software

Even when tethered, the computer can't just write to this portion of the storage. Tethering means that the computer is plugged into the phone - and this means the computer can talk to the phone's operating system. But it still needs to request that the operating system make any changes to the storage.

The iPhone's operating system doesn't allow for the boot sector to be overwritten except in very specific circumstances. Jailbreaking basically involves tricking it into thinking that these circumstances have been met.