Why pay PUBG for playing
Your biggest PUBG questions, answered
PUBG is free in mobile only because Fortnite is free to play and contains in app purchases.
Fortnite success was major threat to PUBG, In fact Fortnite was earning more than PUBG.
So PUBG decided to follow the same path but they can't make pc version free since many people have already bought it and it will unfair to them. So they launched PUBG mobile as free but it contains IAP.
PLAYERUNKNOWN'S Battlegrounds - How to Play