It's a pretty important question, given how much of an advantage it is.. As best I can tell it comes from any combination of these factors:

• intrinsic enjoyment of the activity
  • ideal, but usually not sufficient since there will be aspects that you dislike
• external pressures
  • family, immediate social groups in teammates or friends, broder societal expectations (imagined or otherwise)
  • probably the most common reason that anything gets done: institutional deadlines and/or social pressure
• hyperfixative/addictive tendency
  • of either the person, the activity, or both
• strength of one's sense of purpose/belief in what they are doing
  • being driven religiously or by some ideological mission applies well here
• mindset
  • optismistic, or defeatist looking for every opportunity as confirmation that they couldn't do it?
    • self-doubt

David Goggins is known for his motivational speaking about weathering extreme physical challenges like the Navy SEAL BUD/S selection program and ultramarathons in order to mentally torture himself and test his limits. It doesn't take a brilliant psychologist to recognize from his autobiography that his he is trying to prove himself over and over again in some mixture of internal and imagined external pressure as a consequence of childhood trauma. A grim source of motivation, but evidently effective.

James Scholz, who made daily 12-hour streams during the COVID lockdowns, seems to have been driven by a ideological/spiritual conviction. I've heard internet creators mention how an audience can push them to be more consistent and imagine that helped here as well.

I wrote about Tyler1 previously--he is well known for hellishly grinding competitive games in an iconic "brute-force" style. He's said he becomes addicted to them.