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Because those compilers are mostly sponsored by OS companies to outsource their software development costs in part.

Also notice that all compilers for scenarios where liability is actually imposed, like in physical goods, most compilers are closed source, proprietary, and certified.

Open Source works best for building blocks of software, not for end products. Companies have incentives to share their libraries and tooling, not so much for the final product like a CAD program.

Software engineers find compilers interesting and useful.