Why do phone and tablet have higher resolutions with gooder pixel density and cheaper than computer monitor?!?!
It's the Monitor-Screen. Most GPUs right now supports 1920 x 1080 Resolution. But if you go and buy a standard-affordable-laptop or monitor right now, you still find around most of them to be 1600 x 900 Resolution, unless you provide the budget for the HD-one (1080p, anything below it is not considered full-HD). The thing about 1600 x 900, is it's not even 16 : 9 aspect-ratio! So you will change it to 768 Resolution to downgrade the picture to 720p.
Do not pay for a monitor (or laptop) that doesn't have 1920 x 1080 support(HD-support), it's not worth it. It only costs a few more to go for HD, so go for HD! (atleast!)
The reason why Tablets and Phones have better picture-quality, is because the diodes are very close together(the spacings are very tight) because the screen is very small(this is called having a highier "pixel density" because the screen is smaller). Compare it to a bigger-screen, the pixel-numbers will still be the same(it's the standard) but they have to make the screen bigger, so ofcourse, it will reduce the quality(more space between LEDs, your naked-eye cannot recognize those, but you can towards the quality of picture). Also, the greater the pixel-density, the more vivid the color and more sharper the picture appears. You do not get greater pixel-density from upgrading your screen to bigger because pixel-quantity has to remain standard(the same quantity). This is the answer to your question. Smallest-screens will always have the best quality because of picture-density.
yeah, I hear you...