Actually, Met is very hard to get in plants, period. If you build to a 100g diet, you want a daily met level of at least 0.3% (more, much more, depending on age, breed, purpose) 100g of corn provides less than 0.2% - essentially, to meet Met levels w/ corn, you'd have to eat another 50% more - which has problems of its own. Most of the grains have Met levels (averages of about 0.2 - 0.25%) - once again, you can't met the desired Met levels w/i a 100g ration. Adding synthetic Met and Lys allows Purina and all the others to meet desired Met levels without either a lot of "waste" protein (that is, other excess amino acids the birds just poop out) and with less expensive ingredients overall.
To try and meet targets w/o using animal sources means a lot of certain legumes (like soy), which have their own problems and are costly. Using meals, an industrial by-product, like soy meal, alfalfa meal, etc both addresses some of the problems inherent in the ingredient and the cost issues. Even so, it can be difficult - which is why feeds carrying the "Organic" label can contain synthetic Met and Lys - because its that critical and there are no other practical alternatives.
Even so, total additions of synthetic Met in feed are limited by US law - its a help, not a fix.
