Why is the island of Borneo divided?

Borneo, the third largest island in the world after Greenland and New Guinea, is divided into three countries: Indonesia, Malaysia, and the small Sultanate of Brunei. The Philippines claim the eastern part of Sabah, a Malaysian state previously known as North Borneo, but that case is still disputed.

But why is the island divided?

For starters, there is no real "Bornean" identity. The island is home to many different ethnic groups: Malay, Chinese, and Banjar, just to name a few. So there isn’t anything pushing it together.
And as for the borders of the countries, Malaysia and Brunei were formerly part of the British Empire, while Indonesia was part of the Dutch Empire, and when the era of colonialism ended, the newly formed countries adopted the old borders drawn by the empires.