There are two horns to the answer.
The first is buyers’ remorse. A lot of the colonies in Flat Black didn’t even have a workable plan for success. There were a lot of crackpot utopias, some based on religious prescriptions and others on pre-scientific political, economic, and sociological theories. That they worked at all is because people survived the failure of the plan. Some colonies were set up with assurances of a sound plan by dishonest promoters. There were four colonies planned by diligent government technocrats with no unfounded enthusiasms and no corrupt interests: Mayflower, Paraíso, Navabharata, and Xin Tian Di. In all four cases the politicians cut the funding before the first ship even landed.
So why did people migrate to these places? Selection effect. The people with accurate or pessimistic estimates of what they would be like didn’t go. The people who underestimated the difficulties and over-estimated the comforts and certainties chose to do so.
The second is real options theory. If you make a contingent plan you give yourself options. And if you have to commit scarce resources or turn down alternatives to provide against contingencies you are in effect buy options. If you spend $100,000 and give up a ton of your cargo allowance to have the things you will need to make good the loss if something goes unexpectedly wrong then you have in effect bought $100,000 and a ton worth of insurance. If you spend $100,000 and give up a ton of your cargo allowance to have the things you will need to exploit the opportunity if something goes unexpectedly right then you have in effect bought $100,000 and a ton worth of lottery tickets. Either of those things means having $100,000-worth and a ton less of the things you will need if things go as expected. Having limited resource and limited cargo capacity, people buy only a limited amount of insurance and a limited number of lottery tickets. I have enough insurance against my house burning down that I could build a new house and buy adequate furniture and clothes to be comfortable, but not enough to replace my art and antiques.
Back in the land of the literal, pioneers in Flat Black made some contingency plans and took along some “just in case” equipment. But they didn’t take so many precautions that a disaster would have no impact at all. They planned and equipped themselves to blunt the edge of disaster, not to completely immunise themselves against any possible eventuality.
What you see, in effect, is the result of the plans they had working to some extent, in that 97.7% of the colonies survived at all.