Entering with a full position means putting all your eggs in one basket. If the market goes wrong, your account could bleed heavily, or even go to zero. In trading, no one can claim to be 100% accurate; going all in means ignoring the risks.
Losing streaks are hard to recover from: if you lose, you have no bullets left.
In trading, winning and losing are common occurrences. Even strategies with high win rates can experience losing streaks. If you lose everything in a full position, your funds are ruined, and you won't have money to trade next time. Good position control at least gives you a chance to recover.
Market fluctuations are hard to predict: black swan events come without warning.
Who can really predict the market? Black swan events, such as insufficient liquidity or price gaps, can cause disaster if you're all in. Normal fluctuations could also trigger your stop loss; going all in amplifies the risk.