Because the material in the degrades over time. To minimize this, you also should only store them at I believe 60% capacity when you know you don’t need them. When kept fully charged they will lose capacity faster.
But when you compare them to let’s say a gel, AGM or car battery, you can use 80% capacity of a lithium battery but only 50% of the AGM or car battery. The lithium ones are also lighter, so overall they have a higher usable capacity with lighter weight.
Due to lack of comments so far, here is my amateur understanding.
Li-ion, and almost all other rechargeable batteries, use chemical processes to store charged molecules in the battery liquid. When a current runs through it the molecules discharge and become neutral. When you charge a rechargeable battery, this process is reversed. However, no chemical process is 100% reversable. Therefore, all batteries based of this interaction, which all commercial batteries are, will always lose capacity over time.
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