Bitcoin costs 0.9% more on one exchange than on another. Buy there, sell here, repeat — free money. That is the pitch, and it is wrong for four specific reasons.
1. Two fees, not one
You pay to buy and you pay to sell. On ordinary retail terms that is 0.2–0.5% of the trade, gone before the spread is even counted. A 0.9% gap is already down to about 0.5%.
2. The transfer
The coin has to get from one exchange to the other, and that costs a network fee and time. On a busy network the transfer can take longer than the spread lives. Traders who avoid this keep balances on both venues in advance — which means capital sitting idle on both sides.
3. The order book is shallower than the price
A quoted price is the price of the next coin, not of your whole order. A 0.9% spread with two thousand dollars of depth behind it is a 0.9% spread on two thousand dollars; everything beyond that fills worse and the average price drifts against you.
4. The seconds
Between seeing the gap and having both orders filled, the price moves. Most spreads visible in a table do not survive that. This is the part a static comparison never shows.
What is left
Real cross-exchange arbitrage exists, but it lives on hundredths of a per cent, requires capital parked on several venues, and is executed by software in milliseconds. What is left for a person refreshing a page is mostly the illusion.
You can watch this happen without risking anything. The spread scanner shows live gaps with the fees already subtracted, and the free demo bot trades them on paper — the useful column there is not the profit, it is how many trades it refuses.