Why is port 25 blocked for EC2 and Lambda services?

The following link is helpful in answering how to unblock port 25: Remove port 25 restriction from your EC2 instance | AWS re:Post. However, my question pertains as to why it's blocked.

Doing some research I've found a list of reasons as to why port 25 might be blocked or throttled by hosts and ISPs, but I was hoping for a more official explanation from AWS as to why it's blocked here.

Thank you.