Because they work in different ways. LCDs work by aligning liquid crystals to allow or deny light. Well they hold a crystal in a given state until they are signaled to change it. So regardless of refresh rate, there's never any flicker, it changes only when it needs to. CRTs actually have electron guns that scan over their surface. The gun hits a phosphor dot, it lights up, but then the gun moves on and the phosphor starts to dim out, until the gun hits it again.
For a CRT, the refresh rate is actually how fast the guns move across the display. For an LCD, it's just how often the display receives an update.
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