One reason for using a single CPU is that the number of cores per CPU has become very high and only using a single CPU avoids a lot of complexity around Non-Uni

One reason for using a single CPU is that the number of cores per CPU has become very high and only using a single CPU avoids a lot of complexity around Non-Uniform Memory Architecture where the OS should allocate data to RAM that is "closest" to the CPU it is being used by for best performance.