Why do we use Name Servers and not A Records or CNAMES.?
Since we use Amazon Web Services in a stateless server manner, IP's come and go. An A Record will not work; because that the IP's of the server instances change constantly. Between the end user and the web server is a load balancer that sends traffic to each server instance.
In Route53, it's able to handle a CNAME Alias which is the end point of the Elastic Beanstalk server. Most 3rd party DNS providers don't handle CNAME Aliases like Route53. A regular CNAME pointed to the Elastic endpoint usually fails to resolve.