If the motors were wired in series, the second one would only have access to the voltage and amps not consumed by the first one, making it less powerful. In parallel they have equal access to the power source so both can make full power.
I remember this experiment in 9th grade science; we wired up light bulbs in series, and the last one in the series (farthest from positive battery lead) was always noticably dimmer than the first bulb. Wiring them in paralell caused all bulbs to glow with the same brightness as the first (full power). Home wiring is always done with the receptacles (appliances) in parallel or the voltage drop would be severe at the end of the chain.