TL;DR: That's not all we have. Bones are one evidence point and teach us tons. But there's all sorts of other 'fossils' that are often found with them and add to a far greater story.

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Let's start with bones first.

From the way bones are constructed, we can tell if they have to support heavy weight like an elephant, or let the animal climb a tree like a squirrel, or allow the animal to unhinge its jaw to swallow very large prey like a snake, or show lots of evidence where tendons were firmly anchored to them that indicate an animal with massive strength like modern gorillas. Worn-down and scratched up flat teeth might indicate a grazer that ate a lot of grit.

Then there's the chemical composition in those bones. Certain trace chemicals can teach us about the animal's diet, or about its health. Was it malnourished? Did it live long? And so on.

But there's TONS of other stuff too. Those bones could be found with smaller bones of the creatures that were in the animal's stomach or that passed through into its dung. Its footprints could have been preserved in mud and teach us how long its stride was while walking or running. Certain types of skin and hair are very often preserved too.

So we add it ALL up, and compare it to what we know about modern animals for similarities, and there ya go.