While I'm not an expert in the field, the reason it's not used for standard WiFi networks is probably both that connections would drop out really frequently and

While I'm not an expert in the field, the reason it's not used for standard WiFi networks is probably both that connections would drop out really frequently and that it doesn't scale well (or at all) to large numbers of clients. Directional (line of sight) wireless communication is used in situations where you don't want to or aren't able to lay cables, and have a one-to-one relationship (or one-to-many, as with satellite tv), though.