Before the government SBOM standard that kicked all this off was finalized, I'd asked about this and related items such as reproducible builds. The response I got is that getting honest information out of vendors would be a huge step forward for end customers. Being able to validate that information would require a lot more work. Things like SLSA levels 3+4 (https://slsa.dev/spec/v0.1/levels) go further to prevent lying, at least in situations where all the code can be compiled by third parties.