The amount of rod and cone cells in your eye fill all available space. Introducing an IR sensitive cell type would take away space from existing cells, limiting their usefulness. Having night vision would mean sacrificing something else. IR frequencies would also require larger sensor cells due to their larger wavelength.
As a side note, men generally have a higher ratio of rod cells, allowing them to see in the dark. While women have a higher ratio of cone cells, allowing them to better distinguish colors. Apparently, these tradeoffs were important enough to cause for specialization in our DNA.
As a side note, men generally have a higher ratio of rod cells, allowing them to see in the dark. While women have a higher ratio of cone cells, allowing them to better distinguish colors. Apparently, these tradeoffs were important enough to cause for specialization in our DNA.