Any reason why evolution didn't give us IR vision too?
Marketers of things like CCTV cameras love to sow confusion about these things, as NIR-sensitive cameras are extremely cheap while thermal cameras are comparatively expensive.
Humans are not sensitive to NIR because for the vast majority of human existence, any time there was NIR light there was also an abundance of normal visible light, due to a little thing called "the sun"
NIR sensitivity does not improve night vision. That mostly requires a "tapetum lucidum" or reflective layer, more rods than cones (less helpful when you want to see colors in daylight) or just larger, more biologically expensive and vulnerable eyes than necessary.
Your comment and its grandparent are both excellent, but I would like to nitpick a bit here. Technically, NIR sensitivity does improve night vision through the simple mechanism of "detect more wavelengths = detect more total light". It's not thermal imaging, but it is the reason that CCTV cameras often mechanically move their IR filter out of the way when the light falls below some threshold - at that point, sensitivity is more important than color fidelity, so they accept all the light they can get, even nonvisible. Of course this is also often coupled with NIR illumination LEDs.