Because the battery is limited by real-world physics. Some batteries are capable of some extremely high current. Consider automotive "wet cell" lead batteries. You'll find that they're capable of 1000 amperes or more, especially for turning over huge engines during start. In electronics and physics, many things are a trade off.
Only way to get high current from 9 V batteries is to connect large number of them in parallel, but that would have it's own down-sides. Really, 9 V batteries are extremely poor source of power. If you need current, get rechargeable 12 V battery or some lithium-polymer batteries. They'll be much cheapr in the long run.
If you want super high current, you may have to accept lower voltage, lower battery life, or extremely high cost. A capacitor, as another example, can supply extremely high currents (compared to batteries), but they store charge, and are not a charge pump, as a battery is.
If you "forget about" internal resistance, then the maximum current is infinite. An "ideal" component, non-existent in the real world, can provide mathematically "pure" infinite or zero amounts of resistance, voltage, current, and all the rest. Different battery compositions will have different amounts of real-world "impure" limitations.
Discharging a battery at a high rate for an extended period of time can cause heat generation due to internal resistance, which may lead to a fire or explosion. Monitor the battery pack temperatures carefully and ensure they are cooled as needed. Keep in mind that running batteries at high current discharges also shortens the overall cycle life of the battery.
Consider automotive "wet cell" lead batteries. You'll find that they're capable of 1000 amperes or more, especially for turning over huge engines during start. In electronics and physics, many things are a trade off. If you want super high current, you may have to accept lower voltage, lower battery life, or extremely high cost.