As Cypher says - it’s because they can
. Also it tends to be to see if they can escape out of the top. I was going to suggest adding things to the roof inside the cage to block certain areas of the roof over items he could fall on otherwise. I use rat sputniks and a tunnel tied to the roof leading between a shelf (or house roof) and the sputnik. This blocks most of the roof bars and as it provides a “roof run” they are more interested in using that than other things at roof level!
Alternatively adding a shelf/platform means you could put some harder items under or on the shelf so he just lands on soft substrate.
I have actually stuck hemp mat to a ladder in case it gets fallen on.
One other thing that works is attaching shelves at roof level. I did that when our first hamster was older - didn’t want him climbing and falling when doddery but he liked sitting under his shelves- so instead of removing them I just moved them up so they were flush with the top of the cage inside the cage - so he was still able to sit under the shelf and couldn’t get at the top bars if he did happen to try and climb the bars.