It's a lot like trying to explain the motivation of individual politicians, where they do really compete against each other too but likewise have something of a standard of conduct. They might in extreme cases even kill each other, but you're more likely to see that than see them adopting a genuinely communist platform in order to gain popularity because a communist platform threatens the class and will be met with reprisal from much more than just the little faction competing against them in the election. Communists gaining a foothold anywhere represents a threat to capitalism everywhere by demonstrating an alternative and giving power to workers.

A bourgeois murderer who kills a fellow member of their class will of course potentially be punished by the state, which could be considered reprisal by the capitalist class because they control the state, but you haven't seen true collective action from the capitalist class until you have seen how they respond to communists.