This is understandable as it is the pilots' responsibility, not the stewardesses (thank goodness) to calculate Weight & Balance (as it is known in the trade), and also the dispatcher at the gate plays some part.

It is sort of precalculated beforehand but can change during loading, eg freight turns out not to fit in the rear hold, has to go in the front one, or passengers expected in the rear cabin misconnect and do not get on board. The balance required to be within limits is actually quite fine.

The easiest way to rebalance is to move the minimum number of people (maybe just one) the maximum distance from one end of the aircraft to the other, as happened to you. Looks like it had been nose-heavy on the initial calculation.

The pilots up front are professional folks, they are not going to leave the gate with the plane out of balance. Did you thank them when disembarking for their careful arithmetic ? It's not the easiest to do (if you remember your physics classs in school, bending moments, lever action, and all that, it's the same thing).