The plastic used to make Lego is a byproduct of fossil fuels. It won’t stop being added to the environment until we get off of fossil fuels. It’s not like Lego is digging up oil just to make toys.
They’re also actively searching for renewable plastic alternatives. Like, it’s a huge initiative for them.
In the meantime they just recently started a recycling program where they buy back old bricks, with the stated purpose being their concern for what happens to the plastic when it’s no longer being used.
If you’re looking for a toy company that’s conscientious about their impact on the environment, you’d have a hard time finding better than Lego.