Actually it's not.
$20k might seen like a lot to you at $50k, because your life and expenses are calibrated to that level. It means a little less at $100k, and much less at $200k. I'd argue that if you make it to $500k, a 20% raise is absolutely meaningless because you probably already have enough for your lifestyle needs.
Ever wonder why CEOs get SO MUCH MONEY to switch jobs? This is why. Lower amounts just don't move the needle and prove enough incentive.