There are plenty of reasons why virtually all international negotiations are conducted in secret. For one tiny example, imagine the effect on the markets and perhaps worse the effect of markets on the negotiations.

If the process were made transparent then the negotiations would simply take place somewhere else.

Just remember, when you see a "public negotiation" of a treaty of some kind, that the actual negotiations took place well in advance of any announcements and in complete secrecy.

Do you think nuclear arms control treaties are negotiated in public? Which do you think is more important -- negotiations that could result in the end of the world, or negotiations over tariff policy?