They don’t, but a company built on that premise (private search) that does otherwise would be playing with fire. It caters to users that specifically look for that. I would quit in an instant if that would be the case, for example.

Seriously though even if they don’t track you an adversary could compromise them

This is true about pretty much anything. Unless you host and write the code yourself, this is a risk. It is a risk with searXNG (malicious instance, malicious PR/code change that gets approved etc.), with email providers, with DNS providers, etc.

What solution you propose to this, that can actually scale?