The reason is quite simple: they don't have any to spare.
The AI likes a big military, and that drains a lot of their gold per turn. Also, on lower levels (Regent and below), the AI does not have lots of workers, and doesn't expand as quickly, therefore it has less terrain, and more unimproved terrain with no roads for commerce. Once you get into the Middle/Industrial Ages, they'll have some size 12 cities, a good amount of roads, and will start to be giving gpt.
Note: I play Emperor and in the Industrial Ages, some civs have +200 gpt - if I trade a new tech around, I can get *lots* of gpt. The higher the level, the more cities, more workers, more everything = more gold.