It says to remove the pants' waist string before washing, but why is that?

The string of LACE-UP CARGO BANDING PANTS_IVORY is what is commonly called a waist string. For these pants, it is recommended to definitely remove that string before washing ^[1].

Although the reason is not specifically stated in official materials, two things usually happen when you put clothes with a string threaded through a channel directly into the washing machine. One is that the string gets pushed by the rotation and rolls inside the channel so its end disappears, and the other is that the string hanging outside gets tangled with other laundry, pulling on the string itself or the area around the hole through which the string passes. If you remove the string, both the pants and the string can be washed comfortably on their own.

If you remember the position of the knot or the ends of the string before removing it, it will be easy to match the left and right lengths when rethreading it. When putting it back in, a commonly used method is to attach a safety pin or thin clip to the end of the string and push it through one hole of the channel, and washing only the string separately in a laundry net reduces worries about losing it.