Lifestyle creep is when your spending quietly rises to match every raise, so earning more never actually makes you richer. The bigger paycheck arrives, the nicer apartment and the upgraded everything follow, and a year later you're saving the same amount — or less — on a much larger income. The money showed up; the wealth didn't, because it walked straight back out.
Because wealth is the gap between what you earn and what you spend, not the size of your paycheck. If spending climbs in lockstep with income, the gap never widens and nothing accumulates. Plenty of high earners live paycheck to paycheck for exactly this reason.
When you get a raise, send a chunk of it straight to savings or investing before you adjust your spending — 'pay yourself first.' If your income goes up 10% and your saving goes up with it, the raise actually builds wealth instead of just inflating your lifestyle.
Of course — the goal isn't to live like you got no raise forever. It's to capture some of every increase as savings so future-you benefits too. Enjoy part of the raise on purpose; just don't let all of it quietly vanish into a bigger normal.