Partly because in my experience this:
> Everyone knows the rules and uses signals on bike lanes as well.
… is not true. Mostly because I’ve been on too many separated bike lanes that really aren’t separated—in really dangerous ways—and can’t be without eliminating the car traffic entirely.
As an example: there is a separated lane in downtown Seattle. It runs along a one way road, on the left hand side. The road has frequent signals for left hand turns, with separate stop signals for cyclists and cars. The stop signals for cyclists are smaller and uncommon, so they’re easy to miss. And they’re timed in an unusual order that frequently confuses cyclists and drivers alike.
All of those things led to several cases where I was nearly hit by a car anyway, under conditions that just don’t exist in shared traffic.
And this lane like many others has a beginning and an end, where cars can and frequently do mistakenly enter.
If all of that is not enough, some parts of the lane also has parking to its right, with ramps on either side for pedestrians to access/exit their vehicle, which are often used for deliveries to local businesses, often large deliveries where the delivery person has limited vision.
All of those things are more dangerous, and more likely to result in collisions and accidents (at least in my mostly bike-friendly city) than just normal traffic flow.
Sure this example is particularly badly designed. But it’s not dissimilar to many others I’ve ridden.
This is all not to mention dedicated but non-separated lanes where getting doored by people parking is common and drivers frequently use as a passing lane.
In those cases, the otherwise well accepted (here) “if it’s unsafe beside car traffic, take the lane” is considered an affront by a lot of drivers who expect you to stay in the bike lane. So now you have the added risk of road rage for no good reason.
I can absolutely understand why anyone would prefer these trade offs. But to me they make everything about riding in mixed traffic more perilous, and I’d prefer the risk where I know everyone is following the same set of rules.