Because motors fail. Based on statistical data compiled by the Electric Power Research Institute (EPRI), 47% of motor failures are due to electrical faults/failures. The 47% can be further broken down into rotor problems (10%) and winding problems (37%). The other 53% of failures will be mechanical faults.
Winding defects can occur due to insulation age, contamination, power surges, thermal overload, damaged wire/materials, vibration, and other causes. They begin as energy crossing an insulation fault (such as moisture or contamination), which isolates at least one turn. This generates additional stress and heat across the defect, which progresses until an arc is drawn and the winding fails.