They often do. So while you can buy "cask strength" whiskies at 50-ish percent (iirc last SMWS one I had was ~60% which was ... a little much), scotch is usually bottled at 40-44% (40% is the legal minimum).

As to why they don't always bottle it at the mixed strength - perhaps it's because these higher-proofed whiskies are more for enthusiasts who will explore the whisky at different levels to find what they enjoy best.