The U.S. has had them for years, the problem is that heat builds up inside until either the person in it gets cooked or the cloaking fails.
Yes, it only works temporarily, or requires some additional clever means of carrying the body heat away in a less obvious way, such as ventilation. You can't fool nature (i. e. thermodynamics).
heat builds up inside until either the person in it gets cooked
That's how winter clothing works, in a nutshell. Eventually, all heat generated by the body needs to be shed; with better insulation, that equilibrium shifts towards a higher temperature on the inside.
Doesn't, like, an aluminum blanket do that?
Blocking IR is easy, but it needs to be wearable, rugged, and camouflaged.
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