Other answers have already mentioned the main factors which determine that choice: lower total ESR, lower total inductance, better heat handling capability, etc.
I'll add one more aspect that has been neglected: reliability.
If you have just one big capacitor, once it fails, you are left with a nonworking system. Moreover, a bigger cap can do more damage to nearby components if it fails spectacularly.
Having multiple caps in parallel helps mitigate the effects you have when a cap fails open, because the others will still be there. You could even design the system with redundancy in mind, i.e. adding more caps than the minimum you would need given the other constraints.
There are also issues with endurance against vibrations (this is particularly relevant when dealing with big motors). A single, big capacitor can be stressed mechanically more heavily when subjected to vibrations. The big mass of the cap can resonate mechanically and exert a bigger stress on its terminals or its mounting points, leading to mechanical failure of the cap itself or the PCB it is attached to.
Smaller capacitors, since they have less mass, have less inertia, so they experience and cause less mechanical stress due to vibrations or shocks. Therefore it's also easier (and cheaper) to design appropriate strain reliefs to avoid mechanical stress and shocks causing problems.