I'm not aware of one's personal liability in an accident being in any way limited by personal assets
But in effect it is, because of bankruptcy (debts well beyond your assets can be wiped out). The guideline you asked about can be understood from two key points:
Bankruptcy is not the end of the world. It is inconvenient and stressful, but sets a limit on how bad things can get financially; you can still survive. Given this, the lower your income and net worth, the less you can rationally afford to spend on liability insurance versus other pressing needs.
There is no "typical" judgment amount. You've framed the question on a $100k judgment, but they can easily range over several orders of magnitude at least, say from <$10k for minor injury or property damage to >$10M for death or lifelong care of multiple people (rare, but not extremely rare). This resembles a scale-invariant or Benford's law distribution.
If your net worth is $X, you are primarily concerned about protecting yourself from judgments of the same order of magnitude $X. This is because a judgment much smaller than $X don't affect you much, while a judgment much larger than $X is probably going to bankrupt you anyway. The "scale invariance" idea leads to purchasing (and being able to afford) liability coverage with a limit of order $X.
