If you're making $100k in say, San Francisco, about half your paycheck is gone to federal, state, and local taxes, social security, retirement plan, etc. Say you're left with $4200/mo. Take a look at house prices and rents there. There goes half of that. Now half of what's left on food. Now you need transportation, clothes, a phone, insurance, etc.

Don't get me wrong, it's a pretty good life. And you can definitely spend a lot less money by having roommates or living in a dangerous neighborhood or eating only noodles. On the other hand, good luck raising a family. All in all, it's not nearly as much money as people imagine when they convert it to local currency and cost of living.