Ambient Sensor?
Why does the iPod touch 5th Generation lack the ambient sensor in other words "Auto Brightness"?
iPod touch, iOS 6.1.3, iPod touch 5th Generation Red 32GB
Being just fellow users here, all anyone can say is "because Apple didn't include that feature in the iPod touch". If you want to express to Apple your interest in having that feature, you can use their feedback page:
http://www.apple.com/feedback/ipodtouch.html
Regards.