By statute, anything labeled 'Straight Bourbon' must be matured in new white oak barrels for at least two years.
Some of them are not 'old' at all after their first run, they can be only 2 years old and have a lot of life left in them. Jim Beam, the largest seller is only aged four years, and Jack Daniels (which could be sold as bourbon except for the maple adjunct) is not aged for very long either. They sell an enormous quantity of Jack Daniels.
So we have lots and lots of good barrels that need homes. Some are reused here for whiskies not labeled bourbon. Most go to Scotland, some go to other countries.
The Ozarks supply the majority of the white oak for barrels. The British isles do not have anything like that sort of supply.
Scotch is matured 'the same but different' as bourbon. Scotland has cooler summers than Kentucky, they have a maritime climate, and quite a large variation in topography.
The result is endless variety, which is a good thing for whisk(e)y enthusiasts.
