There’s an old gaming axiom: never split the party. Sure it started with D&D, but it is equally true for every other RPG out there. There are many, MANY reasons why, but really they can be boiled down to just three.
1) It’s Boring For Everyone Else
The GM and half the group are doing something and there is no chance of you showing up and making a difference. More often than not, you (or another player just like you) will take the opportunity to check your phone. Sure you might be half listening, but this is your chance for your mind to wander. Why? You’re bored. It’s best for the group to stick together; you’ll be paying attention the whole time.
2) You Lack the Skills to Do the Job Right
Groups tend to not have much crossover in terms of their abilities. Where skills and abilities to accomplish a task that overlap with someone else tend to have one person doing the job and the other person bored while they sit it out. So most groups tend to spread their skills and abilities out. It has the bonus of covering more areas. Those spread out abilities have the downside of no one can cover all areas. So you might be exceptional at arcane knowledge and can identify something from across the room but lack the ability to sneak over there and steal it with no one noticing. More often than not, you’re relying on another group member to do the sneaking. Split the party and the sneaking member might be missing.
3) You’re More Likely to Die
Here’s the big one. If the GM is expecting combat for a given part of a mission and you all split the party, the GM either has to make the combat easier on the fly or run it as is and most likely cause serious harm to the characters or have them fail. Monsters that are balanced for the whole group can outright murder half. You have half the amount of targets, total hit points, number of attacks per round. So instead of spreading out the damage across multiple people, more is focused on fewer people. Making matters worse, you’re able to deal out half the amount of damage as normal, prolonging the battle and giving the monster more time to harm you, making that half the targets and total hit points even worse. All the while, the other half the group has to look on helplessly while being bored. It is a just a downright bad idea to split the party.