If it takes you 50 years to turn $1 into $2, then you have not won. Put your money in a bank account instead.

If you can do it in a year, you've won. But if you start with $1 then after one year you have $2, then the next year you have $4, etc. It takes less time if you take a $1000 loan and turn that into $2000. Of course turning $1000 into $2000 is harder than $1 into $2, so take a loan as big as you can still double ;)

I'd assume it's because you've spent $100 and made $200 3-4 times successfully, and hence want to up the ante, spend $50k and make a $100k now..

Market conditions change, and many things could threaten your ability to turn $1 into $2 -- adwords prices changing, competitors, macro-economic effects, etc. Further, the more capital you have, the larger your bounty.

In card games or board games, you've created your "engine" -- that isn't victory, but it is a necessary step toward achieving victory.