Why is it that every axon in an adult mammal has a target cell (muscle cell or other neuron) with which it makes synaptic contact?

Bc the axons fail to find a target cell

It is because the axons that fail to find a target cell die thats why every axon in an adult mammal has a target cell (muscle cell or other neuron) with which it makes synaptic contact.

Explanation:

Synapse is basically a structure in the nervous system responsible for the passage of the electrical and the chemical signals from one neuron to another neuron or an effector cell.

Usually the axon is the presynaptic part of the synapse and the postsynaptic part is usually present on the body of the neuron or a dendrite or an effector cell like a muscle cell.

If the axon doesn't make a synaptic contact with the target cell, the axon will die because the neurotransmitters and the chemical mediators will not be released and no action will result on the postsynaptic membrane.