For understanding this you must know basic definitions of screen resolution, PPI and physical dimensions (size of an image).
For example, suppose your screen resolution is 1920*1080 pixels it means that it will show 1920 pixels horizontally and 1080 pixels vertically.
Physical dimensions are the actual size of your image that can be 2*2 inches that mean 2inches wide and 2inches long.
PPI is pixel per inch this means how many pixels in particular physical size. For example, 200 PPI for 2*2 inches sizes the resolution of the image will be 200*2 * 200*2 pixels = 400 * 400 pixels.
Now coming back to your question you want to know why the Image resolution and PPI were same of the image after cropping it?
-->> Well, we can change the size of an image in photoshop by Resizing it through Image> Image Size or just by cropping it, for this Photoshop resamples either downsamples or upsamples the image to obtain the desired output and for altering the image without resampling Photoshop make changes in those three attributes i.e. Image resolution or PPI or actual size of an image.
For example, a 400 x 400-pixel image, has a physical size of 4 x 4 inches and has a resolution of 100 pixels per inch (ppi). To reduce the image's physical size by half without resampling, you set the physical size to 2 x 2 inches. Photoshop increases the resolution to 200 ppi. Resizing the image this way keeps the total number of pixels constant (200 ppi x 2 x 2 inches = 400 x 400 pixels). If you double the physical size (to 8 x 8 inches), the resolution decreases to 50 ppi. Adding more inches to the image size means that there can only be half as many pixels per inch. If you change the image resolution, the physical size changes as well.
Now When you use the Crop tool to resize an image, the pixel dimensions and the file size change but the image isn't resampled.
(Crop tool options) Cropping of an image could be done in varies from those are-
Changing inch size only
Changing inch size and resolution
Changing the pixel dimension only
Changing the pixel dimension and the resolution
Changing the resolution only
Yours is specifically the First option- Changing the inch size only
When you set the physical size of the image in inches in the Crop tool options and you don't change the resolution, the pixel dimensions change. The dimensions change based on the ratio of the number of pixels you drew in the crop selection to the pixel dimensions of the original image. The resolution changes to fit the extra pixels into each inch of the image based on the image's original size.
Hope! This might help.