You do relatively commonly see wastewater piping on the outside of a house in the UK, especially older stock (soil stack from the toilet, waste pipe from sink or bath running into it). This is fine in the UK climate where a normally empty pipe doesn't need insulation. I hear that it won't work in places that get extreme low winter temperatures, but the UK doesn't have winters that cold.

You don't see them on new builds, I think, probably because the pipe going from inside to outside would reduce insulation effectiveness.

Yeah it makes sense for buildings where plumbing was retrofitted.

Otherwise people try to retrofit narrow drain pipes in the walls which are prone to clogging or give you poor flushing performance. Or outside big enough pipes outside interior walls where you get to hear every flush/shower unless you build a box around that. Easier to just run it outside if you can configure your bathrooms that way.