how do they get richer?
if you're just spending bitcoin, not producing anything, nto mining anything, how do you gain more bitcoins? Especially if you already hold a majority of bitcoins?
The rich get richer in fiat just because they are the only ones who can lend money they don't even have, and than charge interest on it. That's why they are so unbelievably rich.
With bitcoin, this scam isn't possible.
Trading is a service. You are providing liquidity to the market, and offering the best prices to sell/buy bitcoin, thus lowering the spread.
Traders also take risk for themselves, and lower the risk for the market, the smaller the volatility is the better for merchants and long term holders.
It is the most important service in bitcoin, after mining.