Happy to answer. The sensor is a 4:3 sensor (more square), a standard that pretty much every camera uses for photography (from phones to DSLRs). One of the main video standards today is 16x9 (more rectangular - what you call 'wide angle'). There are some exceptions in movies, that go even wider, but this is the most common for now and what you are referring to when you say wide angle.
The iPhone (and other devices and cameras) don't utilize the entire sensor for video like they do for photos. A photo shot on the iPhone has a resolution of 4032 × 3024px, which uses its entire sensor to record the image. Videos aren't at the point where they can have this many pixels, so they utilize a section of pixels on the sensor. For 4K, they utilize a 16:9 (more rectangular) portion of the pixels in the middle - 3840 × 2160. For 1080p, they utilize a smaller 1920x1080 portion of those pixels.
In short, some phones like the Galaxy S8 have a 12mp camera and let you shoot 16:9, however it just gives you a crop, reducing the megapixel count to 9mp. Essentially 4:3 and 16:9 are the standards for photography and video, respectively.
Hope this helps!