It's a question of what is the rule and what is the exception. Single parent households correlate extremely strongly with many negative factors, relative to 2-parent households. This doesn't mean that somebody can't live a good life coming from a single parent household, but that on average they are much less likely to do so than somebody coming from a 2-parent household.

And while correlation is not causation, many of these factors are obviously causal. For instance 2-parent households will either be able to provide a much higher average income in the case when both parents work, or a more supportive environment when only 1 parent works.