Well technically they do, but honestly the number of companies that need to worry about redundant DNS is probably .001%. It does matter for the some people, but as someone from SRE once said "100% availability is the wrong target for basically everything".

dig SOA news.ycombinator.com

; <<>> DiG 9.18.16-1-Debian <<>> SOA news.ycombinator.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10818 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 512 ;; QUESTION SECTION: ;news.ycombinator.com. IN SOA

;; AUTHORITY SECTION: ycombinator.com. 900 IN SOA ns-225.awsdns-28.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400

;; Query time: 39 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) (UDP) ;; WHEN: Wed Oct 25 10:52