but setting up a permanent arbitration court for multinational in order to gain a minuscule perceived advantage in fairness in the eyes of the investors (for the existence of which I have yet to see any evidence btw - please do provide some) is totally worth it.

The countries signing the agreement don't finance it.

It is paid as any legal case is paid, and the loser often has to pay the legal costs of the winning party. Some states are even able to earn money from international tribunals, as states more often win these cases then they lose them.

for the existence of which I have yet to see any evidence btw - please do provide some

Will you then provide evidence that this damages the sovereignty of nations? Will you then provide evidence that national jurisdiction is 100% unbiased in 100% of cases? You're quick to ask for evidence after having given exactly none.

It is pretty obvious: A decision made by one person related to one party or a decision made by a group of people half of which relate to either party. The second one is more likely to be unbiased.