Is someone who has $100k reasonable for wanting $1M? Sure, one would struggle to buy a nice house with only $100k. With $1M it is much easier.
Do you think someone who has only $100, whose only thought is how to pay for their next meal or rent check, might similarly wonder “what difference is there between $100k and $1M? They’re both very rich!”
Point is, ambition is like a gas, it tends to expand to the volume of its container. With $400M you have enough for your own and your family’s comfort forever. With $4B it’s less about lifestyle and more about shaping the world to your liking. You might not care about that if that’s 1000x away from where you are right now, but if it’s just 10x away, it becomes more tempting.
What if you want $4B so you can finally end homelessness, or really contribute meaningfully to cancer research, or ending mortality? These are "shaping the world" that aren't possible for someone with a measly $10M, but might take billions to achieve, as well as aligning political actors (who require donations). "Shaping the world" doesn't have to be a bad thing, that's what we otherwise call "civic duty" when we try to shape the world with our votes.
Do you think someone who has only $100, whose only thought is how to pay for their next meal or rent check, might similarly wonder “what difference is there between $100k and $1M? They’re both very rich!”
Point is, ambition is like a gas, it tends to expand to the volume of its container. With $400M you have enough for your own and your family’s comfort forever. With $4B it’s less about lifestyle and more about shaping the world to your liking. You might not care about that if that’s 1000x away from where you are right now, but if it’s just 10x away, it becomes more tempting.