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.
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.
Alternatively, you can buy a sulphuric acid solution with 1250 sp gravity from a battery shop to use as a battery electrolyte. Now all that is left is placing the plates back into the case, sealing the top and filling it with electrolyte. There you go; you’ve just made a battery out of your dead battery.
Lead Acid batteries were introduced back in 1859 and since then, there has not been much change in the composition and manufacturing technique of lead acid batteries. With all the alternative sources of energy being explored and implemented; we are seeing a rising trend in demand of Lead acid batteries.