Distros also represent sets of defaults and software choices (e.g. removing snap). Good defaults can make a world of difference and dramatically reduce time to usability on new installs.

Besides that distros also tend to include theming that’s much more complete and versatile (works at odd UI scales and such) than themes you find online, which can also be of value. Trying to assemble all the components and poke configs in all the right places to get a coherent look is frankly a huge pain in the rear.