They weren't designed with the home in mind, HT didn't exist before the 80's. The ultra wide formats came out in the 50's to compete with TV. The last thing they wanted was for movies and TV to be the same format.
Most movies are made at 1.85:1 ratio which is almost the same as 16:9 (1.7:1).
https://en.wikipedia.org/wiki/Aspect_ratio_(image)
16:9 is the best compromise for the different formats. Otherwise, you would get black bars on the sides if you had a wider TV on most movies.