I'm just going to focus on the UK for this. We have; Southern Rail GWR Southeastern British Rail London Underground Network Southeast All in game and licensed. Why can't we have the logos under the "LOGOS" tab it makes sense. Plus, it means we can create liveries for any operator we want. You could make a DB class 377 or a BR ICE3. Up to you. Also you should be able to use it on any train as that's kinda the point.
There is a mod for PC that adds logos to the livery creator, but it would be great to have that as a base game feature. The mod is available for download on Trainsim.cz
The reason why operator logos arent in Livery Designer is probably because being able to use their logos in different places isnt part of the licencing agreement.