The file selection dialog is not a part of Nautilus. It is either a provided by the toolkit (e.g. Qt, GTK3, GTK4) or by a xdg-desktop-portal implementation. The GTK4 file chooser that is also used by GNOME’s portal implementation supports thumbnails since December 2022 or GNOME 44.
I guess you are using an older (LTS) version of ubuntu that uses an outdated version of GTK.