The vertical reaches are reasonable, the elementals tend to be far too tall across the board. Having its height also be its vertical reach is quite reasonable if the DM decides that the MM height represents the elemental taking a 10 at the base, 16' high stalagmite shape.
The maximum vertical reach (height the creature can reach without jumping) for an average creature of a given size is shown on the table below. (As a Medium creature, a typical human can reach 8 feet without jumping.) Quadrupedal creatures don’t have the same vertical reach as a bipedal creature; treat them as being one size category smaller.
Creature Size Vertical Reach
Colossal 128 ft.
Gargantuan 64 ft.
Huge 32 ft.
Large 16 ft.
Medium 8 ft.
Small 4 ft.
Tiny 2 ft.
Diminutive 1 ft.
Fine 1/2 ft.