When connected in parallel, each cell has to discharge at same rate within a small margin. That means you have to match cells by capacity and internal resistance during manufacturing process. The more cells you put in parallel, the more chance some of them will age differently and cause whole battery to underperform or fail spectacularly.
And why the batteries have to discharge at the same rate in the first place?
That said this is a problem with all batteries which is solved trough rebalancing: https://en.m.wikipedia.org/wiki/Battery_balancing
Without this we wouldn’t ba able to make battery packs with 100s not to mention 1000s of cells and even smaller battery packs would die much much sooner.
A good analogy would be if you have a boat with multiple people rowing and one of them can’t hold the same pace if you don’t adjust for it you aren’t going to hold course and if you don’t let the man catch a break they’ll pass completely eventually.
It's the definition of the cells being in parallel. Since they all have the same voltage difference (as the fronts and backs are all tied together with wire), they have to discharge at the same rate. The balancing dogma1138 was describing is done to allow some batteries to run at slightly different voltages so they all are at around the same state of charge.
Two batteries in parallel have the same C rating! You double the output current, but you also double the capacity. When you put cells in series, you change the voltage but the current capability and capacity stay the same. In both cases the C rating doesn't change[1].
The C rating is just how quickly the battery can discharge itself, so it is entirely dependent on the chemistry and battery conditions.
[1]: there are finer points; more batteries in series lowers output current slightly in practice, and batteries in parallel are quite complex to deal with. Cell size, construction and cooling all have large impacts on the C rating as well but at the end of the day it's simply how quickly a square centimeter of battery material can carry out its chemical reaction safely.
With current batteries, there are just many tiny 18650 cells connected in parallel / series to get the desired power and voltage, right? Are you saying there are some unstated engineering limitations that would hinder getting to the desired "C" by connecting more battery cells in parallel?