>>238For one, competing in Formula One is very expensive. Audi races in WEC (World Endurance Championship) and is very successful there. Volkswagen owns Porsche as well, and Porsche has recently had their own WEC/LeMans program. There are rumors that Volkswagen does not want to have two WEC teams and wants to enter Formula One, but there is no proof of that yet. Lastly, Audi hasn't been involved in F1 for a long time, so why would they enter now?
I think it would be cool to see more companies return to F1, but there isn't really any evidence that that is happening yet.
Also worth noting: Porsche was directly involved in F1 for a very short period of time. From the mid fifties into the early 60s, Porsche was a mildly successful F1 constructor, Dan Gurney, an American driver, scored their only win.
Porsche also designed the TAG engines used by McLaren in the 80s, and the terrible engines used by Footwork in the 1991 season.