moar key fragments
please
please
The key fragment drop rate is non-linear, meaning the more you get the lower the drop rate is. This resets roughly every 30 days after your first key fragment. When you hit the reset point you'll start getting more again. This is basically so that farming them is pointless after a certain amount, and people have an equal chance to get a standard amount even if they don't play daily.
You can read more on Hextech Crafting here; https://support.riotgames.com/hc/en-us/articles/207884233-Hextech-Crafting-Guide
It does feel extremely uncommon though. I'll get like 2-4 the first 2-3 days of the cycle, then get 1-2 more for the entire rest of the 30 days. It'd feel a bit better if it was slightly more linear.
From the numerous posts I've seen, I'd guess that 4 is about what you should expect while the drop rate is high.
If the drop rate decline was more gradual/linear, then people would have to play more games to get the average amount. Having a high drop rate at first then dropping off a steep curve means you can play just a handful of games each month and get the ~4 you're expected to. I personally agree with this system, though I know others feel the same way you do.
this encourages moderate players to play their hearts out once in a while, then abandon the game for a long period of time.
A few weeks isn't really a "long period of time", and how is that a problem?