They bought the HL1 rights back from Sierra in the early 2000s. The real problem is that the code is not in a distributable state and nobody at Valve feels like working on putting it into a distributable state ( https://github.com/ValveSoftware/halflife/issues/1712#issuec... ):

> A while back Valve [had] a partner perforce server that had depots of the source files for both gold source and source that were shared with development partners and some mod teams. This server had a major meltdown and those depots were lost. At the time there was no requests and no activity around gold source development. Resources to rebuild the depots did not exist and still don't so that code is just not available. Once a year someone talks about maybe pulling it together to open source it but once again there are not resources to do the actual work need to package it up. The Sven Co-op team was luck in that there was a package and someone to make it available to them, that does not exist today.

Didn't Valve release a huge update to Half-Life in 2023 (i.e. 6 years after the linked comment was written)?