Why do we require a layer structure?

Sorry if this question might sound stupid, I recently started learning about neural networks in deep learning.
And I noticed that every deep learning network seemed to have fully connected layer structure.
Is these a reason for having neurons in fully connected structure? I mean if an artificial neuron is analogous to a biological neuron then why don't we have mesh networks like neurons make inside the brain?

artificial deep learning deeplearning every layer mean network networks neural networks neuron neurons question reason sound