Because google actually cares about hardware and software security. Read the FAQ: https://grapheneos.org/faq#supported-devices

I believe (as it's open source) there is nothing impeding anybody else to compile grapheneOS in a samsung S10, which would not be as secure, but should still work as any lineage

However I haven't seen anybody try

Because phones have device-specific code. Effectively, each single model is running its own fork of Android. Naturally, Google has no incentive to change this - it makes it difficult to update (planned obsolescence) and install other software (like GrapheneOS).