PCM Sound Dolby Atmos

Apple TV doesn’t support passthrough of audio.

Sound is either:

• PCM Multichannel with Atmos, decoded from Dolby Digital Plus with Atmos.
• PCM Multichannel (up to 7.1), decoded from AAC or Dolby Digital (Plus), or other supported audio format.

As fallback, these options are also available:

• Dolby Digital 5.1, re-encoded from source audio.
• PCM Stereo, re-encoded from source audio.

Play audio in Dolby Atmos or surround sound on your Apple TV - Apple Support

The question is why is Apple restricting the passthrough use to their own video streaming service, Netflix and some other streaming services (Disney?)?

It's really a shame, especially since the passthrough was available some years ago! This is clearly a restriction wanted by Apple and has nothing to do with the technical aspect.

I'm really thinking about changing my ATV4k for some other device which allows it.

Using high quality HDMI cables is critical. I'd recommend something like:

https://www.bestbuy.com/site/rocketfish-8-4k-ultrahd-hdr-in-wall-rated-hdmi-cable-black/3720011.p?skuId=3720011

You have either a Apple TV HD or Apple TV 4K, right?