It's often not the best design but the one that's cheap, scalable and has momentum behind it that wins out.

if I remember my basic chemistry, batteries don't deliver voltages at the level of 10/20/100v directly often, its more commonly 1/2v or 0.5v class voltages. You have to have a much more 'aggressive' chemical reaction to deliver higher voltages. And, the same with current: a single surface between two reacting things delivers less current. Its a function of surface area. Same with capacitance: you sometimes need 'more' surface to big up the effect.

Therefore all you have is stacking it up. parallel or serial, thats what there is to get higher voltages, more current draw, longer life per-cell.

Inside a lead acid battery its multiple surfaces, sub-cells. It's normal. inside almost any domestic battery I suspect its sub-cells, sub-cells all the way down.

A giant roll of surface, to increase the area in contact might be one way of getting "more" in terms of current draw or lifetime. I bet that its voltage remains close to the constant in this, hence Tesla "stacking" up the rolled cells, to boost voltage.

This strategy is one of the remarked upon things when I first heard of Tesla (something like “this California startup is powering their electric car with laptop batteries”) ironically laptops have almost all transitioned to lithium polymer (pouch cells) instead of the 18650s they used back then. Not all car manufacturers use teslas standardized cell technique, as it does have some downsides. I guess time will tell, but I doubt Tesla will abandon this technique any time soon.

Smaller cells are safer due to better containment of thermal runaway, letting them use a better-performing but somewhat riskier cell chemistry.

Using standard form factors and manufacturing techniques made it much easier for Tesla to get batteries off the ground through their partnership with Panasonic. The extra space left by the gaps between cells also has the advantage of being ideal for cooling (battery performance and safety is correlated to temperature).

Separating the cells allows makes it easier to cool them. It also provides more inert metal between them in case of fire.

A certain amount of stacking is necessary to get up to a decent voltage, as others have pointed out. But even "100 brick-sized cells" would be a more dangerous prospect than "thousands of 18650 cells".

The Nissan Leaf uses larger cells [1], each roughly the size of a ream of printer paper. So there are real car designers who agree larger batteries are worth considering.

Of course, the Leaf makes a bunch of other decisions that are different to Tesla - lower price point, smaller battery/reduced range, air-cooling batteries instead of water-cooling, a (now abandoned) battery lease scheme, and suchlike.

[1] https://www.google.com/search?q=nissan+leaf+cell&tbm=isch