Because if consumers genuinely don't notice the sesame seeds are gone, that means they never cared in the first place, and this is a better outcome for everyone. Why were farmers growing useless sesame seeds in the first place? Now they can switch to blueberries or whatever and the world is better off. It's kind of like, thank god those nasty canned olive slices finally disappeared from salads. Also, the company can't get away with raised prices because those buns are still next to regular buns from other companies, and consumers will buy the cheapest one. (Or if inflation is happening, everybody's raising prices, but that has nothing to do with sesame seeds.)
On the other hand, if consumers are actively seeking out sesame seed buns, there is absolutely a point at which they notice and say, what the hell, this brand hardly has any sesame seeds, I'm going to buy the competing brand that still has a normal amount of sesame seeds. The company sees sales are plummeting, and they quickly add back seeds until people are buying them again. But there is an optimal number of sesame seeds here. It makes no sense to double the sesame seeds when consumers don't care.
The idea that a company can keep removing something consumers want to pay for, but they won't notice, just doesn't exist in the real world. In the real world, either consumers do want to pay for it and sales will suffer, or it turns out consumers don't actually want to pay for it and the company gives consumers a simplified product that is still subject to market price competition like every other product, and everybody is better off for it.