Because while making the Lead Acid Battery you will need to open the Battery, cut the welds, make new battery terminals, melt the Lead, Make new welds for making the series connections, you may also need to check the electrolyte and so on. You will need these metal dies for making the Positive and GND plates terminals.
Lead acid batteries are a simple technology, and have changed little since the 1800s. Battery banks for offgrid use are expensive, making home made battery banks an attractive option.
Harvesting from scrap lead acid batteries is a gamble, as any slight ionic contamination discharges the cells, making them useless. If you're determined to do it, make a test cell using a couple of little bits of lead, charge it in the prospective acid, and test its self discharge time.
In order to obtain large capacity in smaller construction of lead acid battery, a large surface must be exposed to the electrolyte, and since the size of a single plate is limited, so to increase capacity of lead acid battery, number of negative and positive plates are connected in parallel.
To make a lead acid cell requires a glass or plastic container, lead roofing sheet that's unused but no longer shiny, 4M sulphuric acid, deionised water, petroleum jelly (eg vaseline) and some plastic to hold the lead plates in place. A hygrometer is used to achieve correct acid concentration.
There are mainly two parts in a lead acid battery. The container and plates. As this battery container mainly contains sulfuric acid hence the materials used for making a lead acid battery container must be resistant to sulfuric acid. The material container should also be free from those impurities which are deterious to the sulfuric acid.