Do those watching on telly really want to watch Real Madrid thrash let's say some Slovenian side, the Danish champions and maybe the German 4th placed finish in the group games that you can mostly predict the final standings of all of them 2 minutes after the draws are made and before a balls been kicked followed by a last 16 tie against say a Dutch runner up before finally getting to the part of the competition where they can play a liverpool?
If real madrid play liverpool first round, you're still getting the big tie everybody wants. You just don't have to wait until the last 5 games to get to it.
It also allows more potential for these glamour ties to be spread out and have more eyes on them. If bayern vs arsenal was in the second round and not the exact same time as city vs madrid, you're not cannibalising the viewing figures for those so called glamour ties half as much.
My "stupid concept" is the same as the FA cup in that it's an open competition, didn't see much complaining when liverpool drew arsenal in the 3rd round and one of thr favourites went out instantly, FA cup in the whole is a better cup competition thrpughout from start to finish relative to who compete in it compared to the champions league who weight everything towards the last handful of games at thr expense of the entire run until the quarters.