Short answer: the fruit needs thickening, and it needs long enough in the oven to bubble.
Fruit releases a lot of juice as it bakes, so without a thickener you get a watery pool under the topping. Toss the fruit with a spoon of cornflour (and a little sugar and lemon) before it goes in — that’s what turns the juices jammy. Make sure it bakes until the fruit is properly bubbling at the edges, not just until the topping is golden; that bubbling is the cornflour doing its job. Leaving gaps in the topping helps steam escape, too.
Related:peach & berry cobbler · glossary:self-raising flour.