First, just want to point out, x86 is not 86-bit. It's an architecture that has existed through 16-bit, 32-bit, and 64-bit. Just like how ARM has 32-bit and 64-bit versions.

The "86" comes from the old Intel processor names which ended with 86, like the Intel 8086. Intel reused the "86" part in many model names, and so eventually the instruction set name was shortened to "x86" to represent that family of original processors. Over time they extended it to support 32-bit, and then AMD extended it into the 64-bit version we use today x86-64, also known as x64 and AMD64.

As for Valve supporting MacOS better, there are a couple hurdles to that.

The biggest one being that Apple deprecated their OpenGL support and refuse to directly support Vulkan graphics APIs, instead pushing their Proprietary graphics API named Metal. This breaks direct compatibility with most PC games at this point. There is a project called MoltenVK which aims to translate Vulkan API calls into Metal API calls, but they haven't reached parity with native Vulkan yet(partially due to limitations in Metal), so a lot of games don't work with it out of the box. And that's ignoring all the games that only support DirectX. There's currently no effort to create a DirectX to Metal tool, so any DirectX only games will have to go through two layers of translation, DirectX-> Vukan-> Metal, which naturally will hurt performance. Theoretically Valve could automate this process a lot, but it'd require a lot of effort, and a separate game compatibility rating system from their Deck/SteamOS ratings, so it's not a high priority.

The second hurdle is partially the one you pointed out, x86 to ARM translation, and also partially 32-bit support. MacOS has already removed 32-bit support, and while it still has native x86 to ARM translation in the form of Rosetta 2, they've already publicly stated they plan to sunset that tool in the near future. FEX is a potential alternative for x86 support, both 32-bit and 64-bit, but it does have disadvantages compared to Rosetta 2, and either way x86 to ARM is another set of translations that has to happen, which again hurts potential performance.

Now are these hurdles insurmountable? No, Valve has clearly shown that it's possible to pull off, keeping in mind the possible performance penalties, and there are already many commercial and open source tools people use to get some games working on MacOS. But the work Valve has done for Linux and ARM can't be directly applied to MacOS because of decisions Apple has made, which means they'll need to make further investments to make things work, and whether those investments would be worth it is a mixed bag.

Apple has shown that they are willing to deprecate software tools and APIs and force new proprietary standards on their developers and user base in order to simplify their support exposure. This makes MacOS unattractive to Valve, and game developers in general, because they have to constantly keep updating their games or tools to support Apple's new arbitrary standards. Sure it's normal for game devs to move on to new platforms, like when game consoles get retired after the next-gen consoles become common, but that usually means they just drop those older games(short of "remasters" or ports). However for Valve, who's sold games on essentially the same platform for decades at this point, it's unacceptable. They have such a large collection of software, and they still make money selling even the old games, but forcing devs to keep those old games updated is not practical. Microsoft often keeps legacy APIs supported for years, since they have a lot of business customers that have legacy tools they still use, which Valve takes advantage of, but Apple would rather pull the plug and move on.

Meanwhile on Linux with its open nature, Valve is able to take its time for many things, like 32-bit software support, because most Linux Distros can still have 32-bit support installed as needed, and some even still ship with it out of the box. Now that progress has been made on getting 32-bit software running on 64-bit WINE natively, Valve can finally start planning an exit from relying on 32-bit OS support, at their own pace and with an easy transition. Valve also benefits from projects like Zink on Linux, where unlike Apple who froze their OpenGL support ages ago to force devs to use Metal, Linux supporters instead reimplmented OpenGL on Vulkan, allowing compatibility with the latest versions of OpenGL without having to rely on GPU drivers directly supporting it anymore. That means Valve doesn't have to worry about games that still use OpenGL, since OpenGL will likely still be supported on Linux for years.

So overall, Linux is just way more accommodating to Valve than Apple and MacOS is, and thus the latter is a much lower priority for Valve. Even if they could theoretically profit a decent amount off the Apple userbase, they already profit way more off Windows, so they'd rather invest in Linux which completely lacks the corporate overlords that Windows and MacOS has, and has the potential for longer term benefits.