I have owned over 100 bikes throughout my life, each is unique in it's own way. However, they all share one thing, they all accumulate dirt on the seatpost ( inside the seat tube ). Why???

I'm not sure of the true answer, however I expect it is a combination of factors:

1. The inside of tubes at the factory hold dirt. This probably transfers to the seatpost after insertion. Keep in mind dirt in the seat tube has nowhere to escape.

2. Removing the seatpost allows dirt to enter the seat tube. Once it enters it will most likely never leave.

3. Touching the seatpost adds oil to the surface, the oil attracts dirt. Eventually gravity and/or rain push some dirt down into the seat tube.