PCs cannot get STR 50No, giants can't throw the boulder I described the distance I described (at least not any edition of D&D I am aware of and definitely not 5e where the weakest giant (hill) starts at 21)
So I will make it simple:
Commoner: max strength 16; max strength with training 18
Strength needed to throw a 10 ton boulder 1 mile: 50
What level does a PC need to be to get a strength 50?