I don't know. I can think of some possible reasons, for example: the people who decide to have a company party do want to be nice, and/or maybe they want to party themselves, and/or maybe they see it as an opportunity to get publicity and network, and/or maybe they do it because other companies do it and so it's just "what is done", and/or maybe they do it because they think it'll raise morale and team bonding.

But I don't know. Maybe someone who has personal insight into the matter can chime in.