Why are Matx, Vec, Point and Size different types?

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In every program I code, sooner or later I bump up into one of these conversions:

• Matx_<Type, N, 1> <-> Vec_<Type, N>
• Point_<Type> <-> Vec_<Type, 2>
• Point3_<Type> <-> Vec_<Type, 3>

...or operations like

• Size_<Type> + Point_<Type>
• Size_<Type> * Type
• Point_<Type> * Type

Why are these four types not just aliases to Matx? What is the design aim behind this?