NS records don't change very often. They can see that I looked up the delegation data about "example.com" once every $CACHE_TTL (~months). They do not get repeated queries at the beginning of every session I have with a website.
A lot of security is about being in the habit of minimizing attack surface. The only thing a TLD (or ISP) nameserver needs to know if I want to use the Doomain Name System is "pdkl95 asked for example.com's nameserver once last month" Instead of giving Google (or whomever) an update e.g.:
;; ANSWER SECTION:
news.ycombinator.com. 300 IN A 209.216.230.240
Separating the requests allows for different cache policies. If you simply delegate the entire recursive resolution work to Google (or whomever), they get to record that you needed "www.example.com" every TTL (5min?). You don't even need to see the domain's NS records in that case, so there isn't an opportunity to choose a cache policy.