Even if it looks considerably curious, inside these big battery packs the main building units are usually the common 18650 batteries with Li-Ion technology. One of the first concerns about it is how dimensions could be reduced using other design. AA or AAA standard batteries also have cylindrical shape, and 18650 ones follow the same rule, only playing with size.
While a cylindrical shape tends to permit an air flow that helps in the case of temperature issues, this characteristic doesn't seem a complete impediment for using batteries with a rectangular prism shape like the well-known and widely used 9V battery. Using the available space between cylindrical batteries, it's possible to add extra capacity.
Another aspect to mention is how EV battery packs account with too many variables, and a main one is vibration and shock resistance without losing contact between the metal parts of the individual batteries. That usually leads to soldering and the use of plastic containers to guarantee that the batteries will stay safely quiet. All these mechanisms obfuscate the replacement of the 18650 batteries renewing the battery pack.
Anyway it's interesting to know that if you buy an EV, there are opportunities for renewing your battery pack as some professional (or anyone with tools and time) could replace every 18650 battery for new ones, even if the battery pack is sealed. The work would be much easier if the manufacturer is kind enough for providing an opening mechanism based on screws though! In fact, we should ask for laws that compel manufacturers to do so for every battery pack carrying standard batteries since it's a bad practice toward consumers and at the same time achieves to avoid a great quantity of waste with noxious consequences to the environment.