I was looking at the details of this jumpstart product:
It claims to be 2 lbs and 14.2 x 12 x 3.5 inches, which I think is for the outer black canvas case, so I would guess the actual device is less than 9.5 x 5.5 x 3 inches. I assume this is a Li-Ion battery.
Conversely, a standard lead-acid battery for a common small car is 9.4 x 5.1 x 8.8 inches - 3 times as big, and maybe 10 times as heavy (This is BCI form factor 51R which is used by the 2015 Honda Civic).
I assume the different battery chemistry accounts for the smaller size of the jumpstarter.
Why do modern cars not use this smaller battery type? Looking at the amazon link it doesn't seem to be a matter of cost - the jumpstarter price is in a similar ballpark to what one would pay for the 51R lead-acid battery.