Can't speak to Belgian law but in the US, the definition of gambling has 3 elements IIRC: consideration (i.e. something of value the player offers up for the chance to win, usually money), prize(s) (i.e. something of value the player wants to win), and chance (i.e. whether the player wins a prize is primarily determined randomly rather than though application of some skill).

Online games dodge these three elements:

-In terms of consideration, the subscription fee doesn't count because it grants players access to the game world, which includes much more play elements than just the drop system, and the player's time doesn't really count since nothing of value to anyone else results from their play time. Note that this gets more murky for Asian-style gachapon-monetized games where the player directly spends virtual currency purchased with real money to roll for virtual items.

-In terms of prize, the fine print of most online games' TOS say quite explicitly that the player has no property rights or ownership of any items they get within the game.(If this weren't the case, they would have to compensate players for the loss of property when the game closed down or if the player were banned.) Since one doesn't "own" a drop, there's technically no prize per se.

-In terms of chance, most likely gaming companies would argue that killing a mob or raid boss counts as some sort of skill. Again, Asian-style gachapon-monetized games are on much shakier ground here.

That being said, it's not impossible that new legislation could expand these definitions to include online games and it's arguable that they should be; you have to admit that there's something unsavory about intentionally exposing minors to gambling-like systems to hook them into playing your game.

(I am not a lawyer and nothing said above should be construed as legal advice.)