u/Consistent-Bag-7370

Question from someone who has been recently introduced to these advance topics

i was reading a book and the text stated that Einsteins GR (classical theory) is applicable upto 10^19 GeV and after that quantum effects are too strong to ignore and we would need quantum gravity description but we use QFT (quantum description) way before those energy scales and it has been proven to be a great theory. Why is it so that we see these quantum fluctuations for both these theory on different scales ? Like why are quantum fluctuations or quantum effects scale dependent.