Public transport planning requires timetable (schedule) information.
OpenStreetMap does not collect such information, nor does it seek to. Such information is often protected by copyright or database rights, so we can't simply copy it from other sites (even if we wanted to) and then redistribute it under our open licence.
Nor is it particularly practical to avoid the copyright restrictions by surveying it by hand - for example, the entire UK railway timetable changes significantly every year, with several other changes throughout the year. We prefer to map things that are reasonably permanent!
In the case of Google, HERE etc., they come to agreements with the train/bus operators to use their data. Since Google doesn't seek to redistribute the raw data, it doesn't matter to them that it's not available under an open licence.
However, there is, of course, nothing stopping people building apps/websites that use OSM data and also negotiate permission for train/bus schedules. Citymapper, for example, uses OSM data for some aspects of the app, but also offers timetable-based planning. Remember that openstreetmap**.org** is not meant to be "a real alternative to Gmaps" - rather, OpenStreetMap's data seeks to fuel an ecosystem of a thousand apps and websites which, combined, are much better than Google could ever be.