Because we've passed peak agricultural land. The land committed to growing crops and pasture used for grazing livestock has peaked. The global population, however, continues to increase. There are methods for maximizing yields from farm land, and we haven't exhausted those, but there's only so much food that can be produced on a hectare of land. We also have to deal with top soil depletion, the risks of monoculture, the effects climate change could have on crop yields, and many other problems.

Edit: I think I need to clarify a few things...

I want to be clear: I am not saying we have reached peak food production. I don't believe that has happened yet, though I think that could happen soon. I'm saying, we've reached peak agricultural land.

There are ways that we can increase the amount of food produced on the same amount of land, which would allow food production to increase even if the total amount of land committed to agriculture stays the same. However, increasing crop yields might require things like using expensive fertilizers, which increase production costs that get passed on to the consumer.

There are other ways to increase food production using the same amount of land, like converting animal pasture land to crop land. But, this would make meat, especially beef, much more expensive (since we would be producing a lot less of it). This could be a good thing, however, since a plant based diet is healthier and much better for the environment. Still, people are probably going to be upset about the price of meat going up.

Capitalism, naturally, also plays a huge role. Many producers are motivated to produce food that will yield the highest possible profits. This, however, does not necessarily maximize efficiency. Land used for raising cattle for instance, produces less calories per hectare than crop land. But people like beef, and they're willing to pay for it, so it's profitable for the beef producers, even so much that some might convert crop land to pasture land, thus further reducing efficiency (in calories produced per hectare).

If we had a different system, we could prioritize efficiency, which would help manage costs. However, even under a different system, we would still have to deal with the fact that land is a finite resource. Even under a maximally efficient food production and distribution system, there would be a limit to how much food we could produce. That being said, it is unequivocally true that such a system would be able to feed many more people than the current one.