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Couldn't it just be a heat dissipation issue? These sensors pack a lot of photo sites into a tiny package.

To first order, long exposures are probably less power-hungry - much of the sensor power is burned in the image readout, and longer exposures mean you're reading images less often.

When collecting light, an image sensor pixel isn't really using up any active power (each pixel is basically a capacitor collecting electrons generated by light hitting the silicon).

I don't believe this is entirely correct. DSLR sensors get very hot during long exposures - to the point where excessively long exposures can introduce noise into the output from this heat. I don't see why this wouldn't apply even more so to phone sensors, with their incredibly high photosite density.

However really long exposures are going to suffer from star trail effects - the earth is rotating relative to the stars, so a long exposure changes stars from a point to a short line, which _usually_ isn't what you want. On a 35mm camera with a fairly wide lens you can get away with ~ 30s of exposure time.

On a pixel phone I think you'd be able to get away with ~ 3s exposure time, but as it's going to get pretty hot over this time I'm not sure how much extra image quality you'd actually end up with.

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