Why don't powerbanks use flat batteries?

Question

Batteries

Why is 18650 used, although a flat one would take up less space and be generally more convenient?
In addition, who bought controllers from eBay to turn a flat li-ion battery into an additional battery for the phone?

Answer 1

Completely used.
For example, I previously used the DasVision BP-6600 on trips, unfortunately now the Akum are dead and it is problematic to replace them - so 18650 would be preferable.
Although the case is healthy and I will probably deal with the issue of pushing 18650 into it :)

Answer 2

PowerCover does not use cylindrical elements, they are just flat, to reduce the thickness. And manufacturers themselves install charge controllers on their exotics. That is why they are more expensive than Powerbank.

Answer 3

Why is 18650 used, although a flat one would take up less space and be generally more convenient?

Flat would be... flatter. :) In fact, the 18650 size has the highest energy density in relation to volume among lithium-ion batteries; This is the main reason why Tesla uses such cans in its cars (although it stacks them in piles). Lithium-polymer batteries are only slightly better, but they are much more vulnerable, and a strong case for them negates the advantage.
Prismatic batteries, of course, are more convenient for a number of tasks, but their dimensions, as Artem correctly noted , are not standardized - therefore they are more expensive, and their capacity is lower than that of cans, and they are produced mainly for specific product lines, and not for open sale. And manufacturers of external battery packs never produce batteries on their own.