see
Pedantic Grognard
In large massed formations, a 15'-to-25' pike is useful because of all the points making a veritable wall of spearpoints.
In a group where formation has been broken (or is a sufficiently small group small it's de facto a broken formation), a 15'-to-25' pike is hard to use because the point is relatively easy to dodge and you'll hit your allies when you try to swing it, or change your grip, or the like. (Which are things you could do with a pike in single combat in an open area.)
The D&D 5 "pike" seems, based on its stats other than weight, to more likely be a long spear short of a "real" pike in length, along the lines of the ranseur, spetum, or partisan (all basically spears about as long as a halberd or glaive).
In a group where formation has been broken (or is a sufficiently small group small it's de facto a broken formation), a 15'-to-25' pike is hard to use because the point is relatively easy to dodge and you'll hit your allies when you try to swing it, or change your grip, or the like. (Which are things you could do with a pike in single combat in an open area.)
The D&D 5 "pike" seems, based on its stats other than weight, to more likely be a long spear short of a "real" pike in length, along the lines of the ranseur, spetum, or partisan (all basically spears about as long as a halberd or glaive).