One good reason: extensibility. Pi is extremely malleable in ways that would probably be harder to achieve if it were in, say, Rust.
The resource usage of my harness is nothing compared to an electron app, and being able to easily customize it is more important to me than saving a few extra MB of ram.
I guess it depends on the harness, for example: chatgpt.exe right now is 1 GB on my windows machine. peseo.exe is 500 mb. and opencode.exe + bun.exe is 300 mb.