Op Amp output question
Probably because you are not using a rail to rail op-amp. Normal op-amps can only drive so close to either rail and bottom or top out.

Check the spec for the TL072

First it is not a single supply op-amp. Though you can drive it +-5V.



From the data sheet it can not drive any closer than 1.5V to the nearest rail. This is in alignment with your chart.