Another issue was probably tracks, producing accurate tracks as they were 40 years ago is a nightmare, and they are of no use for other titles. There’s too many entities, brands and personalities involved, many are already gone, making legality issues for such a commercial project too complicated.
Doing so would involve getting licenses from each driver, each team, each manufacturer, each track and for each advert. When GT Legends was announced in 2005, Simbin/Blimey’s decision to base the sim on the FIA Historic racing championship threw many people, as other sim-racing games had been based on historical seasons… Realistically, there was no way Simbin/Blimey could have reproduced a 60s or 70s championship. (the website is offline, but the content is still available via the web archive)