why do the pictures in plasma/lcd TV's on display suck?

[chicken10der]

I check them out from time to time at the local Best Buy. It's almost always some repeating clip from ESPNHD or a scene from The Matrix. They look pixelated, and there seem to be shadows during much motion. The only exceptions are the ones that are $4k+. Is this a marketing thing? Does the video source they use suck? Are the TV's not calibrated? Are plasma/lcd's just not that great?

I've never owned one myself, and nobody I know has one for me to watch it 'in action'. Any ideas? Do they look okay with just regular cable?