Since c608f056e94e4945c243d2430429470793153dbc, we have introduced OOUI usage on many page views, since the library was needed for new dialog which T217827 required.
The commit message of that patch claims to have optimization: "Do not load any CX module unnecessarily till we are sure that a gray interlanguage link is displayed. For this an init module is introduced". However, we actually load heavy interlanguage JS file, which has dependency on OOUI, when some possible target languages are discovered for user. For this "discovery", we check browser languages, previous ULS selections, etc.
Then, if all languages, determined by such algorithm, have article in their wikis, gray interlanguage links will not show at all, but we'll have script file, which should handle clicks on these gray links, loaded, as well with OOUI needed for that dialog, which will obviously never be displayed.

Also, if gray links are showing, we still don't need to load the script with OOUI dependency, since user may not utilize gray links often, but will read a lot, which just puts a burden on pages to handle actions user may never take. With little compromise of UX, we could load the dialog script (which has OOUI dependency) at time when user clicks gray link.