Because this is the Internet
/thread
As a guy who has cheap charms on his toons, I can say this. If I farmed enough cash to buy a 1.2mil charm, I would not want to do it again. I'd swap around if able. However, I agree that the swapping should not be allowed in general. The intention of the charms was to be a plat sink, if you could swap, it removes the plat sink. As you would only sink it once. You most likely keep getting cash while exp'ing, and that cash would just sit.
Also, how would you track the swapping? I, like most, have my toons on all separate accounts, so I can two-box at my leisure. I have several different e-mails I used, also.
Now, take into account taking over a friends account who quit. Instead of farming cash for his, you just want to swap a charm to his. Would that be allowed in your new system? How could/would that be tracked? The whole idea would just cause head-aches all around. And it would only take one dishonest person to "screw the pooch" on it.
And farming cash is easier now, I am not certain why you don't see that. More exp'd players, with more aa's, and the addition of tomes, kill stuff much quicker. And items vendor for more, that means more cash in the economy. Some people try to auction stuff to others, that is true, but I know that many, including my friends and I, would rather just vendor the stuff at the end of the day, and split the cash. Much easier than waiting on 3pp more from an auctioned item. Part of that is laziness on a part, and part annoyance at the player base that bids 3pp over a vendor price of 500, and thinks it is all good to let me wait 1.5days to get the cash from you, too.
To summarize: higher vendor price + greater toon power = more cash being farmed for similar (or less) effort.
Trading charms = head-aches + heart-aches