I did say "a mockup with a Save button", not "a rock with 'Save plz' carved into it thrown through the devs' office window." Yes, the PO has to care too.
I guess count yourselves lucky you've only had bad POs and not bad POs and bad developers.
I think we are talking about different complexities of projects. In a complex project in a complex environment in a team larger than 2, a mockup often does not cut it. I've seen underspecified requirements leading to misunderstandings countless times. I have never seen a situation where I thought that writing down those 5-15 acceptance criteria upfront was a waste of time. The discussions that follow when a deadline was missed because of misunderstandings feel much more like a waste of time to me.
In a simple project a mockup can be enough, but then what do you need a PO for?
Mockup is great if you are just working on the front end, but what is the save button actually supposed to do? I can figure that out, talk to the users and understand what they want to save, but I feel like that's the product owners job.
I guess count yourselves lucky you've only had bad POs and not bad POs and bad developers.