So why bs? On traditional markets liquidity is about trillions per day.
OK, but the liquidity is easily enough to sell a million dollars or so all at once while barely affecting the market at all. So it's not like 2-4% would become a normal gap when dealing with much smaller amounts of money than that.
Insane in comparison with what? In traditional finance we have such commissions (or may be i mistake?)
In BTC most exchanges take 0.25% at most. More than that and they wouldn't be competitive. Coinbase charges 1.5% but they remove the spread altogether and they still have their dedicated exchange, GDAX.
In traditional markets, the fees vary but I don't think I've seen fees that high before.