Because there is no money in making them closed.

Open weight means secondary sales channels like their fine tuning service for enterprises [0].

They can't compete with large proprietary providers but they can erode and potentially collapse them.

Open weights and research builds on itself advancing its participants creating environment that has a shot at proprietary services.

Transparency, control, privacy, cost etc. do matter to people and corporations.

[0] https://mistral.ai/solutions/custom-model-training