I am not sure in which section to post, please remove if necessary.

We can read countless threads on the ordeal of passagers who try to select a seat on codeshare flights. Codeshare flights happen in a number of situations. In this case, I was looking for a flight from LYS (France) to TYO (Japan), with ANA (All Nippon Airways). I found a LYS-BRU-NRT flight, the first flight is operated by Brussels Airlines and the second one by ANA. ANA does not operate from LYS, so I could not avoid a codeshare flight, should it go to BRU or CDG, before the second flight to NRT.

I can't select the 2 seats for my partner and I on the Brussels Airlines flight, before the 24h window check-in process.

Now my question: why do airlines make it so hard, or often impossible, to select a seat on codeshare flights? What do they gain from it? It seems like computers did not exist, or like airlines want to punish passengers on codeshare flights, why is it so?

Thanks for your input!