Short answer: because the author (not me) did it that way
Long answer (pure speculation):
Because the numbers are not poetic
You are not supposed to know exact numbers (and judging by the frequency of this question, we succeeded at that
We only wanted to convey a message "this is easy", "this is ok" or "this is hard"... which is hard to do by numbers because each mod uses different costs... cost 100 is easy in xcom, but hard in piratez for example.
Symbolic representation conveys this much easier and quicker:
- few symbols = easy
- many symbols or bigger symbols = hard
And some people love the added challenge of figuring out what do the symbols mean by themselves.
Bottomline, there is no particular reason for it... it's just that way and it's not even that bad.
