Back in the days of monospaced fonts, it was common practice to put two spaces after the period ending a sentence to make text more readable. It’s not an issue now that fonts are dynamically spaced, making words appear more “natural” and sentences thus easier to parse, but when every character had the exact same width it was hard to determine the “flow” of a sentence since it wasn’t easy to see where it ended. I remember being taught in kindergarten/first grade to use two spaces after a period, even though we weren’t using monospaced fonts then (to the best of my recollection). That’s why this is an “over 40” thing - it was taught to older generations to accommodate the technology at the time, but nobody ever went back to “unprogram” this from their minds.