Because you're making mash, and mash with skin inside it is....not quite the same thing.

Almost all mashed potatoes I've ever had that wasn't in a microwave dinner has come with skins inside. Do you always use russet or something?

Well, thank you for editing your post to not ask me if I only like microwave dinners.

I guess maybe it's a cultural thing - I've never seen anyone ever make mash potatoes with skins inside, and mash is a dinner staple here(in fact both here in the UK and Poland where I'm originally from).

I encourage you to try it sometime. They mash up wonderfully with the skins on.

You're right. Mash with skin is much better!