Whilst this is true for a lot of cases, it doesn't always hold true for companies which consistently release highly profitable games, an example of such company is Travellers Tales in the UK. With their volume sales, the royalties outweigh paying for an internal team to develop the engine (multiple times over). You can check vgchartz to get some idea of their game sales, and if you are particularly inclined, use companies houe to drill down to get the P&L (although you may need to spend some time getting through their company structure to find the actual P&L which has the right information).

But for a lot of companies, using an existing engine is a no-brainer - even more so when the developer has no idea how well their game is going to do, using something like Unreal effectively means they share the risk with you.