Oh... then I'm not sure what you're expecting to learn. There doesn't appear to be much (if any) correlation between your input and output values. If you provide a 0.0 as input to the network, how is it supposed to predict an output? There's no indication whether the value should be 3.0 or 4.0, so it will always just predict around the mean.

This one input feature is pretty useless. The ideal model is just y=3.5 and doesn't include x at all. If you're able to provide more input features that actually correlate with the output, then you'll get an interesting model.