Why, exactly?
The taxes paid on that 200K, versus the 50K, make the relative spending power of those two incomes quite a bit closer than the 4X multiple would appear at first blush.
Also, "wealth" is very much a perspective. From, probably, a majority of individual perspectives on the planet, nearly every American is "wealthy". I much more associate property ac ulation, rather than raw income, as an indicator of "wealth". Many military officers, for example, due to their years of reasonable income, along with low expenses, coupled with a generous pension, retire "wealthy", although their income would never suggest they would.
My wife and I, on the other hand, each make good incomes (combined, above where you defined "wealthy"). But we have three children at home (who are going to go to college), parents and a brother that need help, and a market that has held our retirement accounts in check for years now. In terms of cash on hand, we are little more able to weather a loss of one, or both of our incomes for more than a minimum amount of time, than a family making far less than ours - AND we are conservative with our money; we hold no debt other than our mortgage, don't have cable or satellite TV, etc.
Wealth is relative - not absolute. The middle class is, indeed, very wide.