The whole point of submarines is that they're stealthy. Active sonor would give away their position. They're basically swimming blind. The only way this all works is the ocean is big enough that random collisions are statistically rare - but there's not much actually preventing them.
It did appear just like that. Shouldn't there be some proximity sensors to avert collisions with massive bodies? After all in the aftermath both subs were rendered mission incapable.
https://www.cbsnews.com/news/whos-to-blame-for-sub-accident/
There is little or no light at typical operating depths. Lidar can work to a limited extent in some water conditions but it's not something to rely upon for collision avoidance.
And, of course, it's an active sensor. An adversary could detect the emitted light.
Is there some spectrum of emissions from an even stealthy sub, some kind of "local presence"? The immediate massive obstacles would result in distortions to such a self-presence profile. So the sensors could be trained to detect such local distortions, almost like some fish have a lateral line of nerves supposedly helping detect current flows etc. [1].
Turns out, this has been modeled already (2007):
[1]:https://www.photonics.com/Articles/Fish_Navigation_System_Re...