To see something in focus the light being emitted off the object must be focussed on the fovea at the back of the eye. Light rays coming from a distant object reach the eye at an almost horizontal angle, the cornea is naturally curved so it refracts the light slightly which is enough for it to be at the correct angle to reach the fovea. If the lens didn't flatten and remained at a more convex shape then the light would be refracted too much and not focus onto the fovea, it would reach the back of the eye at the wrong place causing an unfocussed image to be seen.
In your question above, "I know this makes it refract less light...", it doesn't refract less light, it refracts the same amount of light but at a less steep/sharp angle.