Especially in that age when compute was precious, and scaling horizontally looked nothing like how it does today. This is a time before the Intel 386 which ran

Especially in that age when compute was precious, and scaling horizontally looked nothing like how it does today. This is a time before the Intel 386 which ran at either 16Mhz or 33Mhz :) So it must have been preferable to reduce the workload per email for the server and client.