Why does PowerPoint automatically create a new blank document?

Under Preferences>General, do you you have Show Gallery at Startup checked?

Nope.

You WANT a blank presentation. Choosing a slide layout other than blank will cause your presentation to lock up or freeze when on screen. PowerPoint is a thinking program. By choosing a blank slide layout, you are causing PowerPoint NOT to think and you can design any slide anyway you like. If you choose a pre-set layout (for example, a picture with 3 bullets), it is very hard to make changes to it.
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