There’s also still a fairly standard size for hi-fi separates, and most AV receivers stick to that same footprint.

The bulk of the receiver isn’t really an issue to those who still prefer to use large-ish hi-fi speakers, rather than following the trend of small mono speakers fed lossy audio over Bluetooth or wifi.

Personally, I’ve never liked the ‘tiny speakers plus big boomy sub’ sound ever since it became a trend in computer speakers, and most sound bars just continue that idea.

Also, why is there never the option to turn up the center channel contribution when downmixing surround formats to stereo? Many sound bars are plagued by excessively quiet dialog when playing back certain content. A big bulky AV receiver is likely to give you a lot more control.