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
• social pressures
  • family, immediate social groups in teammates or friends, broder societal expectations (imagined or otherwise)
  • probably the most common reason anything gets done
• 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. As others have said, and what my impression is as well, is that 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 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 his iconic "brute-force" style in grinding competitive games. He's said he becomes addicted to them.