1- I think thats because you can sell trochus for gralats and its the easiest "currency" to get to trade with and also the best price to sell them.
2- Because people want to make profit.
3- People buy all of the rare items in the game then sell them for whatever price they want, and some items are just cooler and more interesting than others.
4- So noobs cant get a bar and be pro straight away, imagine on a game like Skyrim if you got one of the best items in the game in the beginning the game would not be a challenge plus you can just itunes them if you want them that badly.
5- People who have that many tro have been playing for a long time and do a lot to get that much tro its not just by luck.

Because people were getting scammed and it could only be stopped by removed drop able items completely.
Same thing with trading guns.