They could have been two commands that call a single parent command with different options. For example, imagine if cat had an option -N to restrict the number of lines that was printed, and also an option -t to start from the end. head would just proxy to cat -N and tail would proxy to cat -N -t. Combining the three into a single implementation reduces total LoC and opportunity for bugs. Also reduces total maintenance burden.

I believe busybox is like this actually - a ton of Unix commands in a single package. It’s used by Alpine Linux, a tiny and performance linux distro commonly used by docker containers.