I have been building a 'magic' system in a fantasy setting and have a number of ways a human can gain power. Most of these are limited to who can access them by royal bloodline, being god's chosen, intense study, rare ability or immoral choices.
However I have another method where a human can essentially train physically and improve until they can perform superhuman feats, leap buildings, throw boulders etc.
This option would be open to every human so now I need a plausible reason why every human isn't a walking juggernaut and also why people would choose the alternative methods of gaining power.
In short, why would someone choose to not gain these powers, and why would someone choose alternative methods of magic when this option is available?
Edits for clarification:
Once power is gained, it will not decay outside exceptional circumstances so maintaining strength isn't much effort. The time and effort required is exponential as it increases but starts quite easily. Examples from the setting - Character A is a town guard, who deals with petty criminals who sometimes have their own powers. Due to his job and arms practice, by age 27 he has specialized in speed and can maintain a flat out sprint for around 2 minutes and cover a mile. - Character B is a warlord, in his late 50s, has served in multiple wars and trains every day since his teens. His body has adapted to split each of his arms into two at will, and then wield a conventional 2 handedsword in all 4 at once. His specialization is muscle mass and strength, but he isn't much faster than a fit human.