It is not literally how you’re supposed to write. It’s an optional convention that has been increasingly falling out of favor over the decades.

Sentence Spacing (Wikipedia)

The desired or correct sentence spacing is often debated, but most sources now state that an additional space is not necessary or desirable. From around 1950, single sentence spacing became standard in books, magazines, and newspapers, and the majority of style guides that use a Latin-derived alphabet as a language base now prescribe or recommend the use of a single space after the concluding punctuation of a sentence. However, some sources still state that additional spacing is correct or acceptable. Some people preferred double sentence spacing because that was how they were taught to type. The few direct studies conducted since 2002 have produced inconclusive results as to which convention is more readable.