Because it has been repeatedly shown that better social connections help get the right stuff done.

Trust, empathy, and liking each other allows for a generosity in dealings that is very conducive to communication, to problem solving, to finding ways to affect change in the organisation, to train/socialise workers into effective practices, to notice when the work is unbalanced or unaligned with the employee, to correct poor behaviour, and many more reasons.

A competent event organiser could plan to accommodate your introvert preference, and still achieve the prosocial goals.

You could have interactions in smaller groups at a time, have activities/breaks with social recovery (like solo or silent activities, spa/massage/meditation, simulators/noisy activities/activities in heavy gear), have solo parts of group activities (like solo brainstorms or reflective walks), have planned recovery time, etc.

If your social anxiety is that bad, you might need an exemption for health reasons, in the same way a ski trip could exempt someone with a broken leg.

But at least healthy people, including introverts, seemingly benefit immensely from prosocial activities at a workplace.