Because the load is spread equally if there is a difference in the capacity / internal resistance of the battery it can cause a higher load on it than it can safely discharge at that moment.

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.

> And why the batteries have to discharge at the same rate in the first place?

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.