Why can dendrites only receive messages and axons can only send in neurons

I understand that dendrites bring messages to the cell body and axons take messages away, but why does this occur? Why can't they do both?

If I remember correctly, the axons of a particular neuron connect to the dendrites of another neuron (or in some cases muscles). But anyway, that presynaptic/postsynaptic exchange of neurotransmitters in the synaptic cleft is what drives the "message" to the dendrites which in turn, may or may not produce an action potential that travels down an axon and into some other neuron.

---> So in simple words, dendrites bring messages because they are attached to the ends of axons (from another neuron) which send messages.

That's just the general definition. There are neurons that don't have axons and their dendrites can conduct signals. The axon, especially a myelinated one, allows for fast conduction in an "all or none" reliable fashion.