Name one good reason why we can't have a Magma type? We already have rock/ground and water/ice which are essentially the same element

Retypings
>Slugma and Magcargo
Pure Magma
>Magby, Magmar, Magmortar
Magby pure fire, then fire/magma
>Camerupt
Magma/Ground
>Typhlosion
Fire/Magma
>Heatran
Magma/Steel
>Groudon
Magma/Ground

>Strong against
Water, Rock, Fire, Steel, Ice

>Weak against
Ice, Water, Electric