Numerous source games that you could play on Mac before 64 bit was mandatory. Why can't Valve just update the binaries??

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

There are unofficial ports for ARM. They are fully playable from start to end at 2k@120fps (Retina resolutions) on Apple Silicon Macs...

Sure there are minor bugs like flashlight in HL2 not working correctly.

But IMO Valve just don't care about macOS to assign even one engineer to fix this and compile the new binaries.

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.

edit: https://vghe.net/source-engine.html

"Source 1 is extremely old (branching off from the "GoldSrc" engine that Half-Life 1 used which came from Quake 2)"

Q1

They're old codebases that may use libraries that are 32-bit only for things like sound, video playback, etc.

Yeah probably this - Valve experimented with a 64-bit Source engine in around 2005 but that didn't last long, and engine licensees have since added 64-bit support (https://vghe.html/source-engine.html). Not sure where the trickiness lies but certainly in 2005 Source was very different (no Bink video, only DirectX 9 renderer etc).

edit: https://vghe.net/source-engine.html

Huge shout out for Antichamber. It had a wonderfully mind bending learning curve.

The forced 32 bit incompatibility on Macbooks is so weird. My laptop is perfectly capable of executing 32 bit codes. Why does the OS start lying to me that it can't after an update? It's one thing to offer a new product without some backward compatibility, but to remove it via an update? How is it remotely acceptable?

By capable, the hardware is, but the software isn't. IIRC macOS removed the libraries required to run 32-bit applications back in Mojave or Catalina. So it's not really lying. While it is annoying that we can't use legacy software without resorting to workarounds like VMs, it does move the industry forward to 64-bit apps, on macOS at least.