Not sure where you're getting "one total massive cell" from anything I wrote.
Every pack is made of a bunch of smaller batteries. You can't get 400v without batteries in series, from batteries that only make ~3v.
Just saw your last paragraph edit.
It's a car pack, every ounce matters, and doubly so when it only adds complexity, reduces efficiency, and reduces reliability.
And an estimate of weight of the extra interconnect.... let's say it's 8' from back to front of the pack, a 350v pack and a 250kw motor. This means minimum of 715 A.
Busbar that is rated for 700-800A @30c rise has cross section of 1/4"x2". For the 8' length that means we have 48in^3 of copper. That is ~16lbs of copper alone. Not counting the contactors, insulation, etc.