because a lot of these niche $100K/yr products...hit the ceiling at 150-200K. So you have no security like you do when you have 5-6 million in the bank. Anything can happen...Google might slap your adwords account, someone might release a competing product etc.
And starting a lifestyle business with FU money, means that you can tough it out as it grows from 1 sale every other day, to 1-2-3-4-5-6 sales a day.
personally, I'd rather try to run a starup while fighting complacency than while either trying to wrangle investors or working a dayjob. I think complacency would be a lot less distracting.