>>25495283
because you don't properly train your muscles if you do a single set to failure. if you try lifting, you'll see that you have more energy after doing one set to failure than after doing 3 sets of 12 reps.
the absolute ideal would be do to multiple 3-4 sets to failure, but for some exercises it's almost impossible to properly continue until failure, like squats.
if you want to train till failure with no sets, you can! first, you lift till failure with a heavy weight. then you reduce that weight, then continue lifting till failure. reduce that weight again, and lift till failure. reduce the weight one last time, lift till failure. this way you ensure that your muscles are properly stimulated, since one set till failure is not enough.
>>25495291
this is true as well. it's technically slightly better iirc but the benefits aren't massive. if you do it, it's more about being disciplined than about getting muscle.