A few common objections:
Pay-to-win: for those that are focused on skill and competition it sucks to be beaten and taunted by those that beat you because they paid extra to essentially make the game easier.
Balance: many games build their progression curve under the assumption that your stats are improving at a rate that can only be achieved by buying boxes, so the game may be fun but new content frustratingly out of reach unless you pay up.
And in both cases, if you decide to pay up...you probably don't get what you need because of the random and unbalanced design. So you pay again. And again. It is designed to hammer dopamine triggers, so you get just enough of happy payoffs to not stop, but not enough of a payoff to stop either.
Some are fine with the model but object to it hitting children who may not be able to recognize the cost/benefit balance.
Me? I'm glad to see it discouraged as gambling because it is just not fun for the base game. Skill or even just time invested in playing is replaced not only by money, but an unknown amount of money. This distorts game options - think of how many mobile games are time gated constantly with the option to buy stuff (gems, coins, etc) that will let you skip the waiting. Absent the profit, who would make a game where you can play for a few minutes and then have to wait hours to play again? But we fall for it. Or if we don't, we just don't get to play at all.
That's my beef with the "no one makes you play" argument. If i don't play, I'm the one that loses out. If the abuse human weaknesses and make bank (and they do! Unbelievably so), there are no equivalent competing products.
But short story long, that's what loot boxes are and why people object. They aren't a new concept, but they are so successful and profitable that each wave is more extreme.
Look, they say it's for the safety of your kids and vulnerable individuals: it must be good and do not dissent!
The Government has good intentions. /s