Port 443 has to be open. Your staff need HTTPS, your line-of-business apps need HTTPS, your backup agent needs HTTPS. So does malware. Once something is running on a machine inside your network, its first job is to phone home — and it does that over an encrypted outbound connection to a host that has no reputation problem yet, on the one port you cannot close.
Your firewall sees a permitted connection to a permitted port. It is not wrong. It is answering a different question than the one that matters.
The question that matters is behavioral: has this machine ever talked to that destination before? Does it check in every 47 seconds like clockwork, the way a human never does? Did a workstation that has never touched the file server suddenly enumerate all of it? Is a server that normally sends 40 megabytes a day sending 40 gigabytes tonight?
Answering that requires a baseline of every device on the network and a system watching for departures from it. That is what network detection and response is.