Why did they use Unity for Cities Skylines 2? Unity is widely known for performance issues in game dev community right? (I am not a game developer).
Why not unreal engine etc?
While a lot of concepts are transferrable between engines, if you've got a team of experienced C#/Unity developers who haven't touched UE, switching to C++/UE is likely to cost a lot of time/money. A new engine can still be a steep learning curve for experienced game developers. You might be able to pick up the basics over a weekend, but reaching expert levels of knowledge takes years.
That cost increases further if it means restarting a project from scratch rather than building on top of an existing codebase (which will likely include not just the game itself, but also an assortment of tools built within Unity for building content and improving workflows).
Unity's killer feature for many is still its C# support. If you're a small team or solo developer who doesn't need AAA levels of performance, it can be a big time-saver over working with C++.
For that that small/solo team it's also easier to switch engine and since they can't get custom license deals like the big boys having the same language may encourage some to invest the time and resources.
DOTS had a much troubled development cycle; most of what was promised was eventually abandoned, and the key people were laid off. Anyone that bought into it is now stuck in the hybrid-DOTS limbo, where they have to invest substantial amount of time into making the engine features work.