Are you using the free version of Lightroom Mobile? If so, deleting and reinstalling Lightroom Mobile will indeed lose the photos. Photos in the free version are only stored within the app space on the mobile device, if the app is deleted then the your contents of the app, i.e. photos and presets, are also deleted. The only way to get them back would be if you are able to restore the device from a backup taken before the app was deleted.