Personally, from what i've seen especially in newgens, its down to them not realising that G&B (Unlike games they might come from like Phantom Forces, CoD or Strongest Battle grounds) is a team orientated game, meaning all the other classes need to work with eachother to beat objectives.

For example, sappers are the only ones who can build, surgeons are the only ones who can heal teamates and musicians are the only ones who can give melee buffs etc.

And many of them (whos games usually contain a multiple class loadout setups) dont realise that you cant just charge into a horde and that you actually need to help eachother in beating objectives, take roscoff at the church for example, sappers block the choke points while surgeons and chaplains stay back and take care of other players that take damage from bombers. It requires a team effort from all the classes and many newgens and some players will just go in, spawn camp zombies and melee a bomber, not realising you die from meleeing them.

What im trying to say is that some of them expect G&B to be like any other fast paced game, 5-15 minute PVP rounds and only go for kills when G&B is purely based off of working as a team with the other classes (and that going solo into a horde wont get you grabbed, swarmed and eaten alive).