Well, probably (at least) half of those billion people are using one of the large public DNS servers (Google, Cloudflare, et al.). Plus, the answers returned b

Well, probably (at least) half of those billion people are using one of the large public DNS servers (Google, Cloudflare, et al.). Plus, the answers returned by the root servers more often than not include resource records with very high TTL values (e.g., NS RRs with a TTL of two days). These then get cached for that long by the recursive resolvers that are used (directly) by end users. The root servers aren't responding to requests for the A RR for google.com from Joe Schmoe or the MX RR for gmail.com from Outlook running on his desktop -- both of which (without checking) likely have TTLs measured in a two- or (at the most) three-digit number of seconds.