That’s not my experience - my preferred brand* offers 18V batteries at 3Ah, 6Ah and 9Ah. They also have higher end tools that take 56V batteries - either 4Ah or 8Ah.
I haven’t opened them up to check, but surely the higher capacity batteries have more cells.
(* preferred brand because it’s the one I already have a bunch of batteries for… I actually regret choosing that brand)
That said, often when a battery ‘fails’, it’s actually just one 18650 that has failed and taken the others down with it.
Sure… but if you replace that one “bad” cell before it takes down the others, the battery might spontaneously combust and burn down your house while you’re (hopefully?) not home. 18650’s in series have to be the same voltage throughout the charge cycle.