When selection items by dragging the mouse in google slides, you select all elements that are partially within the area you selected.
If you do the same in PowerPoint you only select elements that are completely in the selected area.
The Google Presentations behaviour is undesired. Typically you want to move some smaller objects, but you will always select the main text area of your slide as well.
How to make Google Slides behave like PowerPoint? Use this not so well documented feature: hold ALT while dragging the mouse.
Firefox users: unfortunately ALT is used to hide/show the firefox menu bar, so you get this as an annoying side effect. To disable the ALT behaviour on Firefox, follow these guidelines.