Since the gizzard is for mechanical digestion, shouldn't that happen before the stomach digests the food with acids? (to make the morsels smaller)
The gizzard appears to be a structure that bird inherited from a distant ancestor, as the structure is found in dinosaurs (which birds are derived from) and crocodilians. https://en.wikipedia.org/wiki/Gastrolith
As to why, the bird gizzard is not in a better position, (like say the grasshopper gizzard which is located in front of its stomach), the best reply I think off is evolution is blind. The innovations that appear, are random and further constrained by the features that the animal already has.
It is probably the same reason why rabbit eat their own poop to better digest their food, even though cows have evolved a four chamber stomach to handle the same problem of digesting grass. Or why no animal has evolved a second set limbs to fly even though it would be super useful to keep arms to catch prey and not sacrifice said arms to be made into wings.
In a sense these questions of why didn't evolution design it better is evidence that the concept of intelligent design is rubbish. Designs in animals (and bacteria) can be really really dumb.