Why does CloudFormation require Quantity for certain List types?

I noticed that Quantity is a required field on AWS::CloudFront::Distribution OriginGroups:

{
  "Items" : [ OriginGroup, ... ],
  "Quantity" : Integer
}

Items

The items (origin groups) in a distribution.

Required: No

Type: List of OriginGroup

Quantity

The number of origin groups.

Required: Yes

Type: Integer

Why doesn't CloudFormation infer it from items.count?