More short names fit in your fovea than long names so for the most common operations they are the easiest to read. It’s the same reason the most common words in human languages are the shortest (and programming languages are for humans to use so subject to the same pressure.

Also, Back In The Old Days memory was very expensive and precious so short identifiers were important. Often labels were tightly constrained, for example being limited to six upper-case characters so they would fit in a single word.