The best F1 drivers in the world are certainly in F1, I'll give you that... Even ignoring the pay drivers that aren't in the sport based on talent, you can't ignore the fact that outside of their own discipline, they are generally not the best.
Hamilton - 7 F1 WDC's: Championships in 4 Open wheel categories, Formula Renault, F3, GP2, F1. No wins or titles in any other discipline than open wheel.
Vettel - 4 F1 WDC's: Formula BMW Champion, F1 champion. No wins or titles in any other discipline than open wheel.
Alonso - 2 F1 WDC's: Euro Open Champion, F1 champion. Has WEC and LM24 wins, has Daytona 24 hour win.
Rosberg - 1 F1 WDC: Formula BMW, GP2, F1.
Button - 1 F1 WDC: Formula Ford, F1. Super GT Champion.
Verstappen - 1 F1 WDC: F1.
Kimi - 1 F1 WDC: Formula Renault, F1.
Schumacher - 7 F1 WDC's: Formula Koenig, F3, F1.
Mika - 2 F1 WDC's: Formula Ford, F3, F1.
Villeneuve Jnr - 1 F1 WDC: CART Champion, F1 champion.
Hill, Prost, Mansell, Senna... between them it was only Mansell that got another real accolade as an IndyCar champ.
... so out of 35 of the most recent F1 driver titles (the supposed best of the supposed pinnacle), they've racked up the stunning total of 1 WEC title, 1 Le Mans win, 1 Rolex 24 win, 2 IndyCar/CART titles and 1 Super GT title - that's despite those same drivers having competed in GT racing, NASCAR, IndyCar, WRC, DTM, Dakar and Formula E, Stock Car Brasil and BTCC.
I'm certainly not suggesting that any of the above listed drivers aren't immensely skilled drivers. But their skill sets, clearly honed by their career path to be the best in F1,
only really allow them to be the best in F1.
Off the top of my head, someone like Sebastian Loeb, dominant in his specialist field NINE championship wins in WRC, 16 Dakar stage wins, 6 WTCC race wins, 4 FIA GT wins, a P2 in LMP1 (against the might of the Audi juggernaut, rather than with the aid of Toyotas lack of competition <cough>Fernando<cough>).
The TL;DR is that claiming F1 drivers are the best relies on you believing F1 is the pinnacle to begin with.
It's a restrictive Formula that literally penalizes innovation. F1 is the pinnacle of exploiting loopholes, but it's a wasteland when it comes to genuine experimentation - which is understandable - it's a Formula series... it's literally Formulaic... compared to that of Sports Cars, that are literally prototypes that are allowed to experiment.
It's the pinnacle of a career that starts with Karting, goes to a manufacturer sponsored Formula series, to F3, to F2, and to F1.
It's $2.9 billion in debt, and its blue ribband event is a joke, even amongst its fans. It's risen to prominence thanks to careful, unscrupulous management of an evil-dwarf that would "take a bullet" for Vladimir Putin. Claiming F1 is the pinnacle of motorsport is like saying Donald Trump was the pinnacle of the American people during his presidency - you
could make an argument for it, but any sense-check renders it an offensively stupid position to hold.
I doubt Rossi would trade in 7 MotoGP titles for an F1 drive, I doubt Jimmie Johnson would trade in 7 NASCAR Cup titles for an F1 drive, I doubt Seb Loeb would trade in 9 WRC titles for an F1 drive, I doubt Tom Kristensen would trade in 9 Le Mans wins for an F1 drive, I doubt Scott Dixon would trade in 6 IndyCar titles for an F1 drive and I doubt Stephane Peterhansel would trade 14 Dakar wins for an F1 career.
Ultimately, I agree with
@Liquid, it's a general category, and the disciplines within it, are not directly comparable.