Which is how it should work in principle. Why would parties to an agreement care about each other's internal matters?

Without such a provision you open up to scenarios where a parliament sabotages international treaties by making laws that are in conflict with them.

That's worse than actual official termination of the agreement because the threshold is much lower.

In international law, this is known as monism vs dualism.

Monism says that international law and domestic law form a single cohesive whole. International law automatically applies domestically, and domestic law which contradicts international law is automatically invalid.

Dualism says that international law and domestic law are two independent systems. International law only applies domestically if domestic legislation is passed or amended to make it applicable. Domestic law and international law can contradict each other, and in cases of contradiction the domestic courts will follow the domestic law and ignore international law.

Some legal systems have adopted monism and others dualism. And yet others, like the US, are actually a hybrid – US law is mostly dualist but with a few monist elements.