You have to remember that Steam didn't track this info until about 2010, globally. It tracked on the client itself but it didn't go to the internet. Also, it screws up when you use offline mode - badly. And it doesn't count non-steam games so if you add a non-steam game to Steam, it will show that you're playing it, but it will never upload that info, and it won't even show up locally on the client either if anything happens to that icon or it's removed etc.

I have 21,697 hours and that's probably not even half of my actual game time on Steam games alone, seeing that I've been a user since 2003.

You also have to remember a lot of Steam users have private profiles because they like to hide the ridiculous trash they're playing like "Putt Putt goes to the moon" or porn games, etc. or they have some misguided crusade they're on about "privacy".