Why Camtasia always produce video with a low quality, even if I record in 1280p, for example?
The quality of the images you posted look completely normal to me for what you are doing. You're taking a screenshot and scaling it down. That means that anything drawn as a single pixel in the user interface is going to become less than a pixel wide in the final video, making it seem soft.

You have a few options to deal with this. You can set your screen size to the final video size, or set it so the area you're recording is the final video size. Or you can leave the screen at full size and pan it around as the user moves the mouse. You'll still get some softness due to the compression, but it should be considerably less if you aren't scaling the video.