Can someone explain?

[whyterabbyt]

There's no way to tell how the plugins were originally scanned (they might have been dragged individually onto the window) so we can't remove these root paths.

I might be missing something here, but you have a user-defined list of search paths, so surely anything under those search paths has been found by the search path scan, and everything else hasnt?

[dRowAudio]

I might be missing something here, but you have a user-defined list of search paths, so surely anything under those search paths has been found by the search path scan, and everything else hasnt?

Not necessarily. You might add some plugins during a scan then remove that path from the scan at a later date for some reason.

Even if this wasn't the case I don't think it's a good idea to couple the scan paths to the plugin selector menu. Although the scan paths can determine what plugins get added to Tracktion, they are not co-dependant.

[metamorphosis]

I've looked into this in more detail and it's not quite as simple as it seems. If we create a new sub-menu for every folder you end up with lots of single item folders e.g. "Users" -> "Dave" -> "Library" -> "Audio" -> "Plugins" -> "VST". This is clearly what the existing concatenation was intended to remove. There's no way to tell how the plugins were originally scanned (they might have been dragged individually onto the window) so we can't remove these root paths.

Of course you can - just walk forwards up the tree until you reach the fork level where plugins occur for one of the branches - for the case of included paths.
For the case of drop-in paths, simply start from that folder. Couldn't be simpler.