Those who sell road cars for day-to-day use like Renault, Mercedes or Honda benefit out of branding on their F1 cars, as making consumers associate their brands with a mostly non-existent F1 link helps them sell cars to some extent. McLaren and Ferrari are unique in this respect; their brands are already connected to their F1 legacy. Let me put it like this - anyone who knows F1 potentially knows about the road cars of Ferrari and McLaren. The very presence of these two companies in F1 itself is promoting their road cars as a result, and they don't need to waste the advertising space on their cars with their own branding.
In fact, it is not just that, their road cars conversely promote F1! Imagine potential customers who don't know about F1 listing out supercars to choose from, and wanting to know why some brands are more famous and why they carry far higher value than most competitors (especially a relatively new brand like McLaren). They would eventually end up reading about the F1 history of these brands. And there are groups of people who cannot afford these cars but read a lot - like people who read automotive magazines or nerds who update themselves reading about technology in automotive sector. When they realise carbon fibre monocoque, brake steer, active suspension, active aero, etc are the fruits of technology that trickled down from the innovations in the pinnacle of motorsport, they might want to follow F1.
Coming to promoting individual models like 765LT that way....by doing so, McLaren would be giving away precious space to communicate with an audience who are probably already aware of the product (if not, they will know when drivers do hot laps on this model with some celebrity). That's ultimately ineffective; to promote such a niche high performance luxury product to a customer base outside the reach of F1, they have to use other mediums that would take the product to young, ultra-rich individuals - like a magazine placed in every first-class suite on select airliners or some literature one can find at a golf course or a five-star hotel suite.
Ps. Sorry for the long post.
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