Mostly not a fundamental physics limit—it's largely a materials and manufacturing issue.

Visible-light sensors benefit from enormous silicon CMOS manufacturing scale. Thermal and infrared sensors often use more specialized materials and structures that are harder and costlier to produce. Some thermal sensors also need more complex packaging, and higher-end IR technologies can require cooling, which adds major cost.

Optics are also a big factor. Thermal wavelengths do not use ordinary photographic glass well, so lenses are often made from costly materials such as germanium or other specialized IR-transmitting materials, which are harder to shape and polish.

That said, costs have been coming down. Community answers note that newer manufacturing methods—especially molded chalcogenide glass optics and cheaper microbolometers—have enabled lower-cost consumer thermal products. So the current price gap is not because thermal cameras are inherently impossible to make cheaply; it's because they use less common materials, more specialized fabrication, and far smaller production volumes than visible-light cameras.