They take their lead from their prospective employer, the charismatic Mr Sugar.
They see him behaving like a playground bully and imagine that it must be the way things work in business when all that would really happen is that they'd get a sore face.
It isn't programme I watch as it's subject doesn't interest me. But in part it would be because such folk are deliberately chosen to compete, in order to produce reactions from those who like so-called "reality" programmes. Normal folk wouldn't make for good TV (well what passes as good TV in "reality show" land).
As others have opined, serious editing is done in order to ensure that a certain perception is gained of certain people in the contest - and in the judging panel.
People getting along s not good reality television - conflict is.
By definition, none of the contestants are going to be shrinking violets, and a degree of competition underpins any co-operation - hence the ultimate conflict.
It has been better in previous years. Normally by now you'd see one or two candidates that you could imagine being deserving winners. This year, they've all shown themselves up to be idiots in one way or another (often basic maths!) and not one of the remaining contestants would be a credible winner IMO.