Why neural networks models do not allow for multiplication of inputs?
Using a low-level library, such as Theano or TensorFlow, it is likely that you can construct new schemes for reducing tensors (maybe via some learnable weight vector etc). In TensorFlow, you also get automated gradient calculations, so you should be able to still define a cost function and use existing optimisers, without needing to analyse the new design yourself or re-write back propagation formulae.

The universal approximation theorem essentially states that in order to have a network that could learn a specific function, you don't need anything more than one hidden layer using the standard matrix multiplying 
plus non-linear activation function. So, invention and use of new architectures needs some reason. The reason is usually that these additions improve the speed or scope of what can be learned, they generalise better from training data, or they model something from a problem domain really well.

No doubt there have been explorations of all sorts of variations to the standard NN model over time. The ones that have become popular have all proven themselves on some task or other, and often have associated papers demonstrating their usefulness. 

For example Convolutional Neural Networks have proven themselves good at image-based tasks. The design of a 2-dimensional CNN layer has a logical match to how pixels in an image relate to each other locally - defining edges, textures etc, so the architecture in the model nicely matches to some problem domains.

If you can find a good match from your vector multiplication model to a specific problem domain, then that's an indication that it may be worth implementing in order to test that theory. If you just want to explore other NN structures to see whether they work, you can still do so, but without a specific target you will be searching for a problem to solve (unless by chance you stumble upon something generally useful that has been overlooked before).

To address the question in the title:


  Why neural networks models do not allow for multiplication of inputs?


Some low-level ones do (e.g. TensorFlow). However, this is not an architecture that has proven itself useful, so authors of higher-level libraries (e.g. Keras) have no reason to implement or support it. There is the possibility that this situation is an oversight, and the idea is generally useful. In which case, once someone can show this you will find it would get support across actively-developed libraries pretty quickly since it seems straightforward to implement.