Unity and Unreal are giant resource-hungry game engines. There are plenty of people making games who only need a fraction of what those two provide. Engines like Stride and Godot -- or any number of other smaller engines -- can be easier to work with if you're resource constrained or looking for something more simple.
It depends on the game. Unreal for example is optimized for large teams, and collaboration between different kinds of professionals (programmers, artists, level designers etc.). If you're doing a one-person project relying on procedural generation and pixel graphics, you might be more productive with a much simpler game engine with a much shorter code-build-run loop
Unity is on the same league as Unreal, with the difference to target developers that rather use C# instead of C++, other than that the complexity is the same.
Exactly because of that unless there is a whole team of designers, level builders and artists alongside the core devs, there is more ways to spend time than getting a PhD level knowledge on AAA engines capable of producing Hollywood class 3D movies.
I have tried out Godot and many, many other game engines in the past. They aren't there yet. Even as a hobbyist, I find Unity much more easy to use, and even still I'm tempted to learn Unreal simply because it seems more powerful and well architected under the hood.
They are where Unity 3.0 used to be around 10 years ago.
However Unity now is capable of supporting such kind of productions, with the respective increase in learning complexity and development teams scalability,
Because 99% of people using these engines are hobbyists and want to use something that works for them.
I personally prefer Godot, its node structure makes sense with my mental model of the games I tinker with. And it has enough there that if I did decide to ever finish a game, I could. That's all I need.
If the Bugatti is free, why not use it? Unity is very easy to use, and it gets easier over time. They keep making paid features free, and once ECS is mature (in the UI, I know it's production ready), I think it's going to have a huge positive impact.
Money, obviously. This engine has a permissive license, MIT.
The question is rather what one can conveniently do with it, in the context of the given game, when compared to a big-name engine.
Possibly, for non-demanding games, smaller engines are a good tradeoff between performance/features and cost. Godot, for example, is free, but AFAIK it doesn't have a very efficient culling (yet)¹, so for smaller games, it may still be worth the savings.
Why use Linux or not Solaris? Wait... Solaris is not really that relevant anymore.
I actually get your point but I don't think you should frame the differences as a sales pitch. Most probably people interested are engineers looking for some innovation. So the question is not "why use this" but instead "why is this interesting"
When Discord was relatively new they had a comparison table with Skype, TeamSpeak and other platforms. As a long-time TeamSpeak user, I found that very helpful at the time, because it helped me understand what Discord wants to be.
With Stride it is kind of similar. I don't expect an open-source engine to have the same set of features as Unreal, but they should tell me what they think they are good at! Especially in comparison to Godot, which is a direct competitor in the open-source landscape.
The page talks a big game, and makes big claims. So honestly I'm a bit sceptical :/