My 7 has it, yours does not.
Your iOS seems to be corrupt.
Backup and restore to factory, test, then restore backup
(1)How to back up your iPhone, iPad, and iPod touch – Apple Support
Unlock your SIM PIN if it has one
(2) Restore your iPhone, iPad, or iPod to factory settings – Apple Support
Check settings ad above for auto-brightness. If the setting isn't there it has to be some bizarre hardware fault.
If is now there, restore the backup.
(3) Restore your iPhone, iPad, or iPod touch from a backup
If the problem comes back then you have a weird setting or rogue app, suspect a security app.