You'll notice in the documentation that only ELBs and CloudFront are supported by ACM certificates. You cannot put an ACM certificate directly on an EC2 instance.

While this is a limitation, in practice it's not a big issue. Generally, you wouldn't want to expose your instances to the Internet directly—they should be behind an ELB/ALB for security, performance, and management reasons. If you are serving static assets, CloudFront is going to be much more secure, and performs better at a lower cost.