Has to do with percent of income replaced at retirement with savings, projected spending, and growth assumptions. Fidelity has an article that says the modeled it over $50,000-300,000 per year salary. This is for replacing approximately 45% of income with savings. If I retired today, SS would replace 26% of income.

Most importantly, the general rule is when you make more, you spend more. Car, convenience, house, clothes, food, entertainment, kid activities, vacations, etc. Some of that may diminish when you retire, but to replace a big chunk of income, you want big savings.