Those pesky co-pilots require being paid. They eat. They drink. They must have shelter. They consume goods and services. They even have the audacity to occasionally use the toilet and require sleep every night.
“Really, you only need one pilot. Let’s take out the second pilot. Let the bloody computer fly it” is a quote attributed to Michael O’Leary of Ryanair.
Using that logic, airplanes only need one engine as well. Why waste fuel on superfluous engines?