I don't know how much RB has invested in other motor sports. Perhaps for marketing and commercial reasons, they wouldn't want to pull out of those other activities to focus solely on the F1 power unit. I don't know, just guessing.
Also consider a power unit isn't just an engine block, and a lot of engineering goes into all the other things. Companies like Mercedes, Honda, Ferrari, etc have parent automotive companies that already have tons of R&D into these things, a company like RBR could never compete with global auto manufacturers and their engineering resources. At least these companies have commercial vehicles they can sell and leverage to fund their R&D programs. RBR doesn't even make cars you and I could buy.
Mercedes drew on their the diesel division of Daimler to help them with their F1 power unit, Honda used it's aerospace division, Ferrari has Fiat, and their Fiat owned Maserati MC20, which is using pre-chamber tech. Also consider a Juggernaut of motorsport like Porsche which has huge technical capacity to build engines is shy about entering F1, you can see what a monumental challenge it is.
When you consider what these companies are worth publicly it's amazing Renault is as close as it is, then again Renault is partly owned by the French government, which could explain why funding at Viry isn't an issue. Then you also have to consider the involvement of the petrochemical companies into the power units because the fuel and lubricants are key components to their performance, and who knows how much resources they have involved in the power unit development. Ferrari has had long established partnerships with Shell, Petronas is title sponsor of Mercedes and provides fuel and lubricants, we saw much closer involvement of Esso/Mobil if we remember all the talk of fuel and spec 3's.
McLaren's Petrobras deal pretty much fell through because the Renault power unit cannot use anything but Castrol/BP fuel and lubricants.
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