It'd take a lot of trucks and train cars to move one ship's worth of grain. Consider that highways, train lines, and ports don't necessarily have a ton of extra capacity available, for reasons of economy, that upgrading those takes time, and can be hard to finance if the situation is perceived to be temporary (so, may take significant government intervention to make it happen).
Plus, you go to all the effort and expense of doing that, then Russia hits a few important bridges on the Ukraine side of your routes, and now you're back to nearly-zero capacity.
Plus, you go to all the effort and expense of doing that, then Russia hits a few important bridges on the Ukraine side of your routes, and now you're back to nearly-zero capacity.