Why is the inside of many blue jeans much paler than the front?

Answer: Undyed white weft threads

Undyed white weft threads ✓ — Right. Classic denim is woven from blue indigo-dyed warp yarns and pale or undyed weft yarns. Because denim is usually warp-faced twill, the blue warp dominates the outside while the lighter weft shows more on the inside. The surprise is that jeans are not blue cloth all the way through; they are a blue-white woven structure.

Bleach added after weaving — Not quite. Some jeans are washed, bleached, or distressed after weaving, but the basic front-back color split exists before those fashion treatments. Heddels and Denimhunters both describe denim as blue warp plus natural or undyed weft. Bleaching can exaggerate a look, but it is not the core reason the inside starts paler.

Sweat fades the inside — Not quite. Sweat and washing can change worn garments, but a new pair can already show a paler inside. That comes from yarn placement: white weft threads are interlaced with blue warp threads. The fabric's two-sided look is built into the weave, not gradually produced by the body side fading first.