I know nowadays that doesn’t matter as much due to most DVD players or disc drives being region free, but I recall the time when I was younger in the late 90’s to early 2000’s most discs were region locked based on where you bought a copy (basically the same as buying a Japanese N64 cartridge versus an US one) meaning they won’t work when in another country.
For instance: I’ve purchased the first 2 home alone movies in Japan during my trip back then when they’re re-released on DVD (encoded as NTSC) while I come from a country where most discs by default are PAL so they didn’t work on my normal DVD player, having to purchase a multi region DVD player just to watch them. (This was before streaming sites).
The other answers are missing out on the key reason: licensing.
Copyright law differs between regions, and so different groups need to be paid when the video is “sold” to a customer. Before the early 2000s, this was even more the case than it is today; the US hadn’t yet tied its own definition of copyright to all its trade agreements.
End result? Selling a US DVD in Japan would have been illegal; not because of the region restrictions but because the people who had to be paid to play it in that region hadn’t been paid so the DVD was effectively a bootleg.
Yeah but the main reason for all those copyright law differences comes down to “greed, money, and control” as the other replies explain.