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.
What is stopping my manager from doing a 20% raise today, then replacing me in 3-6 months with a lower cost person? How do you build trust or guarantee a 2-5 year timeline attached to that raise?
I thought I had trust with my manager, but when I converted from contract to FTE (required), they rejected my request for a small raise and tried to lowball me. They eventually settled on the same pay rate, so 2 years with no raise. Only to "adjust" my salary up in year 3 because 2 people quit. I'm still looking at an easy 15% increase and less stress by jumping.
It's very disheartening to not be even given the chance to fight for more money for my team.
Be kind to your manager. If you're seriously interviewing around at least drop hints in your 1:1s so your manager can start the ball rolling about your raise internally.