A cut off tool, like the name implies is only meant to cut off objects. The cut off tool can be cordless since it uses less power at a shorter duration. It also spins at a slower speed, typically around 6000 RPM and usually uses a 4- 4.5" disc.
An angle grinder, while it can be used to cut with is primarily used for grinding. Grinding takes much more power, is continuous and is at a higher speed (typically 10,000 - 12,000 rpm for a 4.5" disc).
Thus, angle grinders are corded and are not found in cordless.