People will agitate for their interests, beliefs and values. Any outcome to negotiations will have winners and losers. Even if people benefit from some deal there is probably another plausible deal where they benefit more or a deal where they get just as big a benefit but their hated enemy gets nothing or is actively harmed. Without the ability to have private negotiations and present the final deal on a take it or leave it basis your negotiations will at best be slower in teaching an agreement.
Negotiations are hard, the more parties there are the harder they get. Public negotiations have very large numbers of parties, even if most of them can’t vote.