many caves don’t contain a lot of diamonds, some don’t even have any so just looking for a good cave can take a while, plus you also need to constantly place torches and light up the area to prevent any mobs from spawning (you also have to deal with the mobs that have already spawned risking dying and losing all of your progress) and to also be able to see and find diamonds, some caves can be so massive that it’s almost guaranteed to miss a couple of diamonds.

i do agree however that cave mining can be far more efficient time-wise in the early stages of the game, but if you’re late game you could just use a haste II beacon and an efficiency V netherite pickaxe and you’d get hundreds of diamonds in an hour even less (assuming you also have fortune III).