Convenience, adds impressive abilities, failure is an option
Seeing that there plenty of such toys that have the traditional helicopter layout, there is no "must" involved here.
A typical toy helicopter (image source)
But quads and other multi-rotors have a few of great advantages: symmetry, commonality between the engines, no need for complex variable pitch of the rotors. This makes manufacturing cheaper, and the control algoritms much simpler, allowing for much more autonomy and self-control.
It also allows for some pretty impressive moves that a normal helicopter layout may struggle to replicate (although given the right pilot, it sure can give the quads a run for their money).
Also — as opposed to full scale helicopters — it is not a disaster if a drone fails; failure is an option. Hence the problem of keeping all rotors running flawlessly and perfectly synced — which is an absolute must for a full-sized multi-rotor carrying people, but also a major headache to achieve, often to the point of being prohibitive — is not a problem for the drones.