Source 1 is extremely old (branching off from the "GoldSrc" engine that Half-Life 1 used which came from Quake 2)
Valve seems to be making a pretty strong effort to distance itself from working with that engine where possible (though it's unclear if their lagging tentpole title Team Fortress 2 will get the Source 2 treatment)
It's possible that rebuilding these games for 64-bit Macs specifically would encounter an undesirable amount of bugs
Valve ported Source to 64-bit Windows in 2005 (https://vghe.html/source-engine.html) but there were bugs, and it seems like they just let it fall by the wayside (I don't think it's been available since 2006, maybe 2007 at the latest).
And yeah, it seems like they've been abandoning Source 1 for a while now. Valve recently hired Joshua Ashton who has been giving Team Fortress some love but it seems like the focus is well and truly on Source 2 now.
Valve seems to be making a pretty strong effort to distance itself from working with that engine where possible (though it's unclear if their lagging tentpole title Team Fortress 2 will get the Source 2 treatment)
It's possible that rebuilding these games for 64-bit Macs specifically would encounter an undesirable amount of bugs