You need to separate out ppi which is a metadata value used in printing, by the printer driver, to decide how many pixels will fit per inch of printed media from pixels. For on screen use ppi is irrelevant and just a number stored alongside the image data.
For example : A 3000 pixel by 2000 pixel image at 72ppi is exactly the same as a 3000 pixel by 2000 pixel image at 600 ppi. Both will contain the same image information. However when printed on paper, the 72ppi version will measure 41.6 inches by 27.7 inches wheras the 600 ppi version will measure 5 inches by 3.3 inches. Both though will contain exactly the same image information - no more, no less. On screen they will measure whatever the screen pixel density sets the image to and both will zoom to the same degree as image ppi is not used for displaying on screen. Hence why Export strips out the ppi data altogether.
Note : The value in Preferences > Units & Rulers > Screen resolution is used to show rulers at actual size and ensure that View > Print Size works correctly. It should be set to the pixel density of your actual screen (mine is 109ppi) yours will be different.
Dave