You can get it from anywhere.
Getting it from a plant changes absolutely nothing.
Here's the part you are missing:
Do you know why, if it's so easy to get, they most commonly source it as a petrochemical?
It's not because oil is an especially great source, it's because when you do so you save energy. Because that oil has energy, and you can use it while making your product.
To source your atoms elsewhere, then burn oil to gain energy needed to modify your product is backwards in environmental friendliness. You are adding another layer of inefficiency to oil usage.
> There are functional curves to look at here with more variables than what the raw count of joules that the process itself uses.
That's what they want you to think "look we are making environmental progress". It's not so. Sourcing carbohydrates is easy. Energy is hard, and they've made no progress there.
Using more energy in a way that can be ramped into more sustainable sources (if it currently is not sustainably sourced--but, again, Lego seems to be at the least offsetting their energy use in toto) with fewer externalities is not the same thing. There are functional curves to look at here with more variables than what the raw count of joules that the process itself uses.