Because, as I said, it's a feat on screen and onscreen feats take priority when they're not contradicted as the MFTL+ feat is, which...
Okay, I'm assuming you watched the movie, so you'd know the context of the sub rel scene. She's searching for the location of the emergence. You'd think that in this scene she'd be running at MFTL+ speeds, then why the hell is she moving at only sub rel, which is, as stated by others, thousands of times slower by comparison. She knows the urgency, she knows what's at stake, so why would she move that slow if she could move at MFTL+ speeds. If it's shown she's consistent with Captain Marvel's flight speed in another movie, fine, fair, the feat can be used, but as of right now, we have one major on screen feat for her speed and its no where near that level.
As for this, another instance of something that's on screen. When I say onscreen, I don't mean literally shown on the screen I mean something shown visually or said in the movie, the primary source for these characters