Research is a small community. Within your field most people know each other and know who's contributing reviews and who isn't. If someone's being a jerk in the community everyone knows about it.

I've been in my field for 20 years. I only know a handful of people who are contributing reviews.

(Some have have told me told me directly, regarding a specific paper they reviewed. Some refuse to be anonymous in their reviews.)

It's not just "being a jerk". There's a whole range of network effects which also confound the issue.

Suppose I am the world experts on $SPECIALIZED_TOPIC, but it isn't a hot topic, and only three people in the world publish in the area. I'm a reviewer on all ten papers published on $TOPIC in a year.

Suppose my co-worker works in $DEEP_AI_TOPIC, with thousands of papers published per month, and reviews 100 papers in a year.

How does my employer weigh information about our respective peer review contributions?

Or, suppose my co-worker and I are equally competent, but my co-worker was in grad school with the editor, so the editor is more likely to send papers to my co-worker.

Also, the industry researchers I know tell me their respective employers have increasingly de-emphasized community service, including in conference organization, where there's no issue of anonymity.