... and that's exactly what they are doing, adding more lead, not (necessarily) more fragile if comparing two starter batteries. They do weigh more.
To quote the car battery faq:
Car and Deep Battery Frequently Asked Questions (FAQ) Section 7
What do I look for in buying a new lead-acid car or deep cycle battery?
web.archive.org
that small in stature battery with high cold crank [amps] will have many very thin lead plates just to get the necessary surface area to make that big cold crank number. It will have the lower volume of electrolyte to provide the cooling necessary for long life and the greater capacity to run the [electrical] systems on the car. All of those thin plates will corrode away and fail long before expected putting the high performance battery's life below that of the lower CCA rated battery with the lower cost. Your best rule-of-thumb is, if it meets the OEM (Original Equipment Manufacturers) recommendation, buy it. Look for the highest reserve capacity [RC] battery at the correct CCA (Cold Cranking Amps)."