Look at the record.
Ford (in the form of Jaguar ) in the sport succesfully in the form of Cosworth for decades. They start a team ( Stewart, then Jaguar ) don't do much and they quit.
Honda very succesfull engines, buy a team, don't do much, and quit.
BMW in the sport successfully with Brabham in the 80's. Returned with Williams and things looked good. Bought their own team, a bit of success and then they quit.
Toyota start an F1 team. Don't do much. Quit. KERS was an opportunity to push their hybrid drive presented to them on a silver platter, and they didn't do it. It was possibly the least tenuous KERS-to-road-car link available, and it didn't happen.
Renault could very well be next ( superb result as engine supplier in '90s. Great success as a team, but as the dust settles on the silverware...it's time to go)
Whereas Mercedes have it JUST right. They write a big cheque to Ilmor every year who make F1 engines and put a merc badge on it.
If they succeed "Wow - look how good the Merc engine is"
If they don't "Oh dear - it must be the Chassis"
Everyone's praised Mercedes because of the results of Brawn and Force India. If they'd have been crap, we would just say "Oh - the McLaren chassis just isn't up to it".
As a full up constructor - it just makes no sense. If you're doing crap ( and not everybody can win all the time ) there's no one to blame but yourselves, and you're spending a lot of money damaging the brand and producing negative publicity. Other than Ferrari, only 4 times in the championships history has a car and engine from the same factory won the constructors championship - and even then, two of those (Renault) were an engine from Paris, and a car built in Oxfordshire. The anomaly is Ferrari who've had massive success- but let's face it, they're not exactly an ordinary company They could put a prancing horse on a dog turd and still sell it for £110K, come last in the WCC and still have 100,000 rapid fans turn up at Monza, and still look cool.
Private teams with good engine backing. It worked for Lotus, McLaren, Tyrrell, Williams, Brabham, Benetton and Brawn. Indeed old man Tyrrell would be very proud of what his team has become. Via BAR and Honda...it's now back being a private team with manufacturer support, a genuine racer in the form of Ross at the top.
Renault will almost certainly be next, and the sport will slowly move back to being a sport.