There could be various factors but the optimistic me wants to leave cheating on the bottom of the list unless proven true with physical evidence.
CSGO can instill good habits into people who play this game.
- Reaction Time
- Crosshair Placement
- Flicking Ability
- Tracking Ability
- Spray Control
And like someone mentioned already, when you do your placement, your MMR is basically a wildcard, which would typically start you off on the easier end of players, and then progress higher or lower depending on how well you do.