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.

I've worked in many projects where I met people only over phone and WebEx or similar. It was always pretty "dry" and tensions rose quickly whenever shit on one end hit the fan. Typically after just one personal meeting (kick-off, war-room, whatever) that changed completely. You start to joke together, you let your guard down more easily. You talk differently, even on the phone and in virtual meetings then.

I also often enough witnessed people bitch at each other over some formulation in a pull request or a comment in a chat room. In person they completely behaved differently and were able to talk it through.

Not everyone ticks the same, but in a large team you can be sure to have at least some people who have an easier time reading body language than hearing nuances in a voice filtered through a microphone. And for these people it's then less stressful to work stuff out in person.