Why cant I overfit this dataset with my neural network?
It is usually good to start with a small model because you can then evaluate the contribution of adding layers, etc.
Also, Boston dataset is a popular dataset so there are several tutorials showing good neural network architectures, like this one.
Concerning your model, here are some notes.

The use of sigmoid activation is likely to worsen results, since sigmoid function compresses values between 0 and 1, while you are trying to predict outputs between 5 and 50.
Instead of sigmoid, you can use ReLU activation, which has better convergence properties for inner layers
You can standardize your target data to reduce the variance of your data and control the mean, this usually improve a lot regression models. In this case, a sigmoid activation on your final layer would be a good choice