And reasonably so, since it will take quite some time to build an entire triple-A-grade game engine from scratch (not just in Rust, but with any language! Though building it in Rust will make things much harder since they need to first build entire ecosystems from scratch...)
I think Embark Studios ultimately do want to use Rust for their projects and are pursuing small R&D projects to go in that direction (rust-gpu and also their Kajiya renderer) - but they're first and foremost a game company that needs to release games for profit, so made the pragmatic decision of using UE5 at the meantime.