Why some tracks have preview url Search but not int Tracks?

[New user]

When I request some tracks via Search, all tracks have a preview_url, but when I request the same track by ID in Tracks, the song doesn't have preview_url. Doesn't make sense, or the track doesn't have preview_url or it has but not half. Happens with some songs like TQG, Real G, Marisola Remix, etc.