Couldn't really find any other websites offering it, so I reckon gambling on it must not be that popular.
Or am I missing something here?
I was surprised too, competitive competitions with PUBG would mean a lot of players and a lot of people be spectating. I haven't tried searching for it but I think there would be a lot of players to watch from (from a point of view of a viewer). I think the popularity of betting on it is really hard to find, probably just with other people and no actual site accepts it.
It's not the kind of betting that you are thinking here in PUBG! Here the players are betting on their own capability. There is no third party betting on any player here! It's very simple, you pay 20 bucks for registration and receive 15 bucks for each kill you complete in the game. Your stats are visible after the game and the organizers will verify it after each game and pay you back.
From the organizers point of view, it is hugely profitable venture. From each game the profitability is 26.5% on each game!