Soaps had lye that would've washed away the seasoning on the pan. Today's common dish detergents do not, so this is just advice that used to be helpful, but it isn't anymore.
Some people are stuck in their ways, never really understood why they shouldn't use soap in the first place, or didn't notice that the soap they grew up with is different from today's dish detergents... I think there's a lesson to be learned here about re-evaluating our assumptions every so often and not doing things just because that's how they've always been done.
This red herring is presented with such delicious haste every single time cast iron cookware is mentioned. I think it's because of some pop-science cooking channel on the internet
While I will not pretend to know all the reasons in fact that this practice has reached such common use, washing your cast iron pan with soap won't strip the seasoning that is already set, but it will strip all of the unset oils left over from cooking that are on the surface. If you spend tons of time maintaining cast irons, you will find that the single most important practice to building a smooth surface that never sticks is leaving a thin (towel dry / buffed, leave too much and it turns sticky) film of oil on the surface of the pan after every use. If you need to or want to use soap to remove the (again, unset non-seasoning) layer of oil on the surface, you can dry off the pan, add some oil and buff it with a towel before storing. Using some heat during this process, but not smoking the oil is necessary to expand the pores of the pan (or make the oil more viscous and better fill the static surface of the pan, whichever mental model helps you sleep). This process takes extra time and effort to achieve the effect you already have with a well seasoned cast iron after cooking: Food debris washes right off with some light scrubbing and water, leaving a clean surface with a thin layer of oil on the top that has nicely filled all the cracks on the surface.
Some people are stuck in their ways, never really understood why they shouldn't use soap in the first place, or didn't notice that the soap they grew up with is different from today's dish detergents... I think there's a lesson to be learned here about re-evaluating our assumptions every so often and not doing things just because that's how they've always been done.