Because the International Air Transport Association has produced guidance (https://www.iata.org/contentassets/6fea26dd84d24b26a7a1fd5788561d6e/passenger-lithium-battery.pdf) that most (all?) carriers accept.
Why have they produced that guidance? Because of the risk of fire if lithium rechargeable batteries are faulty or damaged. The bigger the battery, the more energy they may store, and the harder any such fire will be to contain or extinguish.
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