It's a natural habit of business. Trading is a system that is not see as secure by the market. There may make profit or loss depends on market, risk, time, types, overall depends on the situation. Trading loss commonly depends on the failure to risk identified, lacking patients, world economy, inflation, etc. To prevent this before getting on market one should have to well understand about market.
Actually OP was wondering when everyone knows and agrees that BTC is going to grow in future and the price always increase later if not sooner then how are people still having loss with trading.
I believe the basic reason why people are having loss despite knowing BTC is going to go up is because of two reasons mainly.
1 They are trading altcoins: We all know how destructive and implosive the altcoin market is and it takes one mistake to lose all your capital to a scam coin.
2- Day Trading: A lot of people sell and buy BTC at different prices and earn so sometimes when during a bearish run people have to execute trades at loss despite knowing that if they can wait a week or month they might get better value, they need that money to next trades so they have to accept loss.