In my experience, the problem is two-fold. First product managers/owners don't consider the URIs, so it ends up not being specified. They say "We should have a page when user clicks X, and then on that page, user can open up modal Y", but none of it is specified in terms of what happens with the URIs and history.

Then a developer gets the task to create this, and they too don't push back on what exact URIs are being used, nor how the history is being treated. Either they don't have time, don't have the power to send back tasks to product, simply don't care or just don't think of it. They happily carry along creating whatever URIs make sense to them.

No one is responsible for URLs, no one considers that part of UX and design, so no one ends up thinking about it, people implement things as they feel is right, without having a full overview over how things are supposed to fit together.

Anyways, that's just based on my experience, I'm sure there are other holes in the process that also exacerbates the issue.

As a UX designer, this is a failure of the UX designers, IMO. If you're a UX designer for web, you should be aware of web technology and be thinking about these things. Even if you don't know enough to fully specify it, you should be able to enough such that you can have conversations with a developer to work together to fully spec it out.

That said, I've also worked with some developers that didn't like intruding on their turf, so to speak. Though I've also worked with others that were more than happy to collaborate and very proactive about these sorts of things.

Furthermore, as a UX designer this is the sort of topic that we're unlikely to be able to meaningfully discuss with PMs and other stakeholders as it's completely non-visual and often trying to bring this up with them and discuss it ends up feeling like pulling teeth and them wondering why we're even spending time on it. So usually it just ended up being a discussion between me and the developers with no PM oversight.

Web developers should make it a habit to ask/require URL structures be part of the spec.

I've had people be surprised by the request because its something they don't usually consider, but I've never had anyone actually push back on it.