tl:dr; we like things short. Better to say "first come, first served" than to say "the first patrons to be properly presented shall be the patrons who will be serviced first" and extend that shortening to all concepts, including words like "go, be, am" etc.
AFAIK the reason is because English (and most of the other latin-alphabet-based-written-languages) try to capture, as best they can, the sounds that we make using the fewest number of characters.
In English, we have gotten really good at using a few characters to be really expressive, so the word length is shorter when written.
Additionally, as a language, it has generally grown to where we attempt to make our words more concise at any one point in time. Here's an example of a book which references that which I could thumb through and find a reference to this phenomenon if you like. I know that he condensed many other tomes in his writing in that book to glean out the kernels and to make it a good introduction for other readers. I suggest checking his selected bibliography if you would like even more information on the topic.