A manager going to HR with "cost of living increase please" vs "I have a valued member of my team who is dropping hints theyre unhappy about pay" are very different things.
You're presuming all employees are like yourself. Not everybody is motivated by money. I have team members who are well paid, know it, and turn down large increases elsewhere due to the projects they have excite them, etc.
Like, your manager in their mid-30s might value steady employment, a good salary and good benefits, whereas their low-mid 20s report might value fast promotion, salary growth and relocation potential. So if their request to move from London to Seattle got denied, the manager might think it's not a big deal, while for the report it's a complete deal-breaker
If that happens multiple years in a row, it's not just possible, it's likely.
Of course, it's your manager's job to make sure you are more comfortable talking to this about them than you are interviewing with strangers, which honestly is not a high bar to clear.