It’s possibly at least partly a case of journey planners assuming you want the fastest possible option, and not taking into account the premium Heathrow Express

It’s possibly at least partly a case of journey planners assuming you want the fastest possible option, and not taking into account the premium Heathrow Express charges? I’m not sure why TfL would want to actively push Heathrow Express over their own options, as they don’t have anything to gain, as it’s run by the airport and GWR / First Group, not themselves.