Why cant an ant get as big as an elephant?

No arthropod (insect, arachnid, crustacean or centi/millipede) can get truly large, because of two main reasons. The breathing system of arthropods consists of a network of tubes through which pure air flows to the organs. In larger animals this system would not work. It would have to fill the entire body to reach all the cells and there would be no room for organs! In a small animal, even the organs furthest away from the tubes still get oxygen because they're closer than in a large animal.

The second reason is their exoskeleton. A small insect can be covered in one, but a larger animal could not handle the weight of a proportionate exoskeleton. It would be too heavy.

Now, in the Carboniferous period, millions of years ago, there were giant arthropods - millipedes almost as long as a car, dragonflies as large as eagles - but the air was far richer in oxygen at that time, because animals were just getting onto land while plants had been there for far longer, so they'd built up massive amounts of oxygen that were only just starting to be breathed in. Also, arthropods ruled back then, not threatened by reptiles or birds, so their exoskeletons weren't as thick, either!