I wonder if this is because older folx don't change the default settings on their stock keyboard so it keeps adding a period after every space automatically.
With some typewriters and older word processors, you needed to hit space twice after every period. It's possible some people of those times didn't understand the "after a period" part and just got used to double spacing everywhere. This likely got even worse when, as you note, some phone keyboards began automatically inserting a period on double space inputs.
Did you need to hit the space twice? I thought it was just a style thing where people thought it was better to separate sentences in an attempt at easier-to-read text.