It's not "a sinister plot to poison babies" though. It's the same as any other company which has something to sell - they want to sell as many and as much as they can. They want to maximise their market. Getting you to buy the products earlier IS in their interest, because it's such a small window that baby food applies to.
You might well say well, if nobody cares, why don't coca-cola and heinz beans start marketing their products at babies? Answer - they don't need to, because their products are aimed at anyone from age 2-80 or whatever the life expectancy is now. But realistically an extra 18 months of buying their product is nothing in that window. But if parents are currently weaning their babies at 6 months and using baby food until, say, 9 months, and they start weaning at 4 months instead then that's an extra 2 months added to the 3 they already use - practically doubling the length of their customer base.
Obviously this is simplified. The point is it's not about it being a sinister plot, it's about money, same as anything else, it's just people get pissed off with it because there are health implications which never seem to be as important to the companies as the money is.