Q:

Why are most multirotors built with an even number of rotors?

The most common types of multirotor are quadcopters, hexacopters and octocopters (with four, six and eight rotors respectively). 
Are there scientific reasons why most drones built with an even number of rotors?

A:

Yaw authority and ease of mixing is generally the answer - since yaw response isn't necessarily the most linear, being able to have N number of motors contribute to yaw in one direction with the same number in reverse is the simplest motor output mixer configuration.
Tricopters do work and are relatively common among odd-numbered multicopters, but once you've exceeded four rotors, there aren't many meaningful performance improvements possible for the added complexity of requiring an odd number of motor outputs and doing a custom mix for that.
There are some slightly exceptional cases such as a forward puller motor mounted perpendicular to an otherwise conventional even-motor copter frame (see FliteTest MilleniumFalcon), but these aren't really a convention odd-numbered motor copter configuration.

