What causes the threads on bike pedals to become stripped?

Having never had this happen, and having had one bike for over twenty years, I can only think that the pedal must never have been tightened up properly, so it was moving and wearing out - otherwise the pedal/crank connection is actually pretty solid.

It is something that can be hard to tighten, especially as one thread is clockwise and one counterclockwise, which can confuse people.

It probably means that the pedal was never installed correctly: either it was not tightened enough or it was cross-threaded.

It's actually not that hard to install a pedal incorrectly. If you miss perfectly aligning the threads, it's called cross threading. It's possible to cross thread the pedals and still have them feel like the pedal look like it is installed properly.

However, if you look closely, you'd see that the pedal isn't perfectly perpendicular to the crank. When you pedal, you'll feel a pop as the pedal goes around when your foot crests and the pedal (due to not being parallel to the crank) has to change orientation.

Over time movements of the pedal relative to the crank will slowly strip out the threads on the crank, on the pedal, or both.

I could speculate that your raleigh may have been purchased at a department store, and that it wasn't assembled by a professional. They didn't install the pedal correctly.

I would guess that the pedal was cross-threaded. Either that or it simply wasn't properly tightened to begin with. (In fact, a right-hand pedal may have been force-fit into the left crank or some such.)

In any event, the crank arm (in addition to the pedal) is likely damaged beyond use. The only possible repair of the crank arm is to install a "Helicoil" repair kit, if one can be found of the correct size (and rotation).

In my experience the best way to fix/prevent that issue is before you install the pedals make sure to apply a water proof grease like this to the threads.

It costs about 3 dollars and will ensure that the threads will be less likely to cross thread and most importantly the pedals won't seize or stick to the crank arm the next time you change out the pedals.

If the damage is already done you can buy a 9/16" 20 tpi die to re-thread the pedal like in the image below and a 9/16" 20 tpi tap to re-thread the crank arm.

Generally, the bolts one should use are titanium bolts. They do not wear out easily and they will last for ages! The only downside is that they may be quite costly, meaning about 6 euros per pair. The pedals' threads probably were damaged because there was not enough grease or good quality grease on them. This causes the two threaded parts joined together to have friction and day by day them getting destroyed.