Connecting 3 phase ac motor
The centre-point of a star connection will be at zero volts when a load is perfectly balanced.  With mixed loads (like a whole building), you cannot hope that the load will always be balanced, because there may be lots of single-phase loads which are being switched on and off at various times, and you care very much about your phase voltages being constant, so you have a neutral.

With a motor this is not the case - nothing[1] is going to cause the centre point to be at anything other than zero volts, which means that if you connected a neutral cable it would carry no current, and hence be a waste of time/effort/copper.

[1] Well, nothing probable.