Why does the Samsung S9 not have a 4k screen and will we ever see one on Samsung phones?

Why has Samsung not upped their screen resolution to 4k on their mobile phones yet? I though the S9 would defiantly have a 4k screen. Do they not want people to buy these phones or what? A Samsung S7 & Samsung S8 are practically the same phone apart from a small upgrade, but there's nothing to shout home about.