Why salesforce does not support Roll-up Summary on Account for Contact as it is only supporting one standard object which is Opportunity ?

I have checked for Roll-up Summary ( <https://trailhead.salesforce.com/en/point_click_business_logic/roll_up_summary_fields> ) and salesforce says that its mandatory to have Master-Detail relationship between objects then only you can have Roll-up Summary field on parent object.

According to above, how is Roll-up Summary on Account is supported for Opportunity object ?

Please check these screenshots: [![enter image description here](https://i.stack.imgur.com/8HssV.png)](https://i.stack.imgur.com/8HssV.png)