The current BFL chip has an on die temperature sensor, they just aren't using it. If I had to guess it's a cost issue; it's cheaper to toss a couple 20 cent temperature sensors on the board that it is to properly mux and read 16 temperature diodes.
My guess is that the reason to use external sensors is that the internal ones are too noisy and very inaccurate. The cost is indirect: they can't afford to design the de-noising hardware/software and can't afford to properly characterize and calibrate the internal sensors.
It is actually quite common problem: many SoC chips have on-die temperature sensors but use external ones because otherwise they wouldn't meet the time-to-market goals. This includes even very-well capitalized entities which already have the methodology and the tools available.