Why are there only a few non-empty fields in the WHOIS records?

The WHOIS/RDAP system has undergone many policy changes recently. Most importantly, WHOIS data are heavily prone to data protection regulations such as the EU's GPDR, resulting in missing or masked data contents even in the case of non-personal data. In the case of country-code top-level domains, the data content strongly depends on the policies of the national Internet registries; some countries tend to hide almost everything. Importantly, the relevant dates (registration, update, expiry) are almost always available.