That comes from typewriters, before kerning was a thing. Each key press moved the paper an equal distance, so every single character was evenly spaced. Even narrow characters like i or l had the same amount of space on each side of them. Monospaced font is easier to read when sentences end with a double space. But with modern kerned fonts, the double space is pointless.

Phones sub in a period for simplicity, so you don’t need to reach for the period key. It doesn’t actually include the double space at all; it removes the first space and replaces it with a period. If you’re “supposed” to double space after each sentence, why does your phone remove that first space?