Unfortunatly that is like saying all cars are the same, and if you tune them all up, they all would go just as fast, and drive the same as each other.
One manufacture doesn't have a magic wand to wave over their product and make it cheaper than the rest, they use cheaper parts.
Use Plasma as an example. There really are only a hand full of companies out there that actually make the plasma "glass". So now you have the potential for slight quality differences, and price differences there. The rest is video boards, power supplies and input cards, and in some, tuners. All things that can affect the quality of a picture, and price.
You always get what you pay for.
Pricing has come way down on plasma, in part because they are building more to meet demand, but the demand also brings out the cheaper brands to help fuel the market. No one wants to be the last one on the block to buy a HDTV do they? They also know that the "average" consumer doesn't know the difference between a good picture and a great picture, they have been watching their old Sony or Zenith TV for the last 10 years with the brightness and contrast level set way too high. (when was the last time you adjusted your TV to a proper NTSC calibration, or any new settings?)
Heck, I wouldn't be suprised to find out that even with plasma Sony raises the red levels, RCA does the blues and others doing their little tricks. For those of you not sure why... we all have favorite colours, they know this and by tweeking the red, green or blue colour just slightly, their picture may appeal to you more than another brand.
Thats enough for now.