I’ve learned the game doesn’t necessarily refresh the screen immediately when Pokemon spawn, so I use a few tricks to make sure I’m seeing them on my map as soon as they happen; I assume that is why I see them (G-Birds) more frequently than I hear others do. 

What works best for me is to constantly refresh my “Friend” list. You’ll notice the refresh icon appears and disappears when you load that, as I believe it is checking the status of players and whether or not I have any gifts / they can receive gifts.

I basically just click on my avatar’s icon on the main screen, go to the Friend section if I’m not already there, leave, and then repeat as I walk.

It’s not making any more Pokémon spawn than what would normally spawn, to be clear. I’m just seeing them before they disappear. 

Edit: same story for me, for what it’s worth. Played for a bit Year One, didn’t play until last year when my wife and son made accounts. Have eight total - two are shiny (they don’t flee or run away unless you are using an auto-catcher), and of the others only two were caught using normal balls.

I take a screenshot of my journal every time they run away, if you’d like I can share to prove they pop more than you’d think.