Purpose of Nginx container vs. cloud based elastic load balancers?

Hello, I'm currently learning K8 and was curious why Nginx is one of the most popular containers on docker hub and a prime example of container orchestration in a lot of k8 courses. I don't know a ton about dedicated web serves like Nginx or apache. Since most companies run on public clouds like AWS/Azure/Gcloud with built in elastic load balancer services why would you not just route through that to your front end containers orchestrated through k8? Why go through the hassle of orchestrating another layer of containers? Any help in understanding this or better explanations of deployment workflows through kubernetes would be great.