Seeing as they're expensive and the individual pieces have tons of value, people often resell them or donate them. Or even for those who toss them in the garbage, they are buried in a landfill. But it's hard to think of any kind of plastic that actually gets reused more than Legos are. My nephews are now playing with the same Legos I played with 40 years ago, together with some new sets. Not a lot of toys (or objects generally) you can say that about.
They're not a major concern for polluting the environment. The kinds of plastics that are of concern for pollution tend to be food-related (bags, utensils, lids) which people actively use and lose and discard outdoors (and blow away from trash cans etc.) and plastic commercial fishing equipment which is similarly used, lost and discarded in the ocean.
Where the FUCK are they all? microplastic in the environment, no. it wast towed outside the environment to another environment where there are just fish, birds, sea and lego...
anyway... as much as I love lego, they need to curb their roll on the plastics they put into the environment. No, not that one. The one where the lego doesnt fall off.