There's a number of reasons, but the two main ones are:
- when compared to SD digital teevee is: Poor colour depth compared to the number of pixels.
- For a person used to analog teevee (PAL, or NTSC) - and not digital teevee, a major weirdness is the digital compression used. Digital teevee is compressed
Compared to SD digital teevee - No increase in colour richness.They increased the number of pixels shown per cm (or inch), but did not increase the richness of colour displayed.
On PAL for example, you may have 422 colour depth - which is 8 bit colour. If you come from the computer world you may think that is 8 bits per channel colour (RGB 888). However in the broadcast world they use the YUV colour space, and this is usually only 8 or 10 bits per pixel. 4 bits Y, 2 bits U, 2 bits V == 8 bits per pixel.
If the colour depth is the same, then why does it look weirder in HD? Imagine a rainbow going from Left->Right. PAL would have 768 pixels wide, and 'HD' 1920 pixels wide.
The same number of colours would be on the rainbow line, only the HD one would be kind of zoomed in.
The error is zoomed in - or enhanced.
There's other issues like how blue ray only does 8bit colour, and then there are cameras that only output 8bit colour... or worse. Then the equipment in various distribution centers, or the center which puts the little logo in left hand side of the screen, might have different resolutions (say one operates in YUV 422... 8bit). Then you have the compression codecs, and settings being used by the broadcasters. Then the link from your broadcast box to your LCD can down-sample to 8bit. Finally the LCD internally may only do 8 bit, or the display only 8bit. If any one link in the chain from the camera to the LCD is 8 bit, then the final result will be 8bit (or possibly even worse, if the conversions are done badly... eg a fast YUV422->RGB->YUV422 conversion are not lossless). Or the camera natively uses bayer colour space, which is converted to YUV422 by the interface, and then to RGB888 in the computer, then back to YUV422 in the encoder, through to the teevee which is shown in bayer colour space again right at the end. Each conversion to a different colour space is lossy.
Other reasons why it looks weird are enhancing the error of the frame intervals. Eg, film at 24fps somehow shown on a screen that only does 50hz. We've gotten used to things been shown at one frame rate compared to another. The movement is different - and human beings can easily tell the difference.
Teevee is now very complex, and a lot of factors complicate what is being shown to you. Everything, from the speed of things being shown to you, to the colour depth used, the bandwidth used per teevee channel, to the size of the display to the type of technology being used to show you... even the glass on the screen is different.
Why does HD look weird?
All of those things have an affect, however the lack of enough extra colour depth is one of the main things.
Compared to analog teevee - digital compression.
For a person used to analog teevee (PAL, or NTSC) - and not digital teevee, a major weirdness is the digital compression used. Digital teevee is compressed, so they can fit more channels in the space available to them. The compression changes the image so to the human eye... little detail is lost.
That's the aim anyway. However, compression is never perfect - and many people can see the weirdness in the compressed images.
The world of teevee is weird - but digital teevee is even weirder.