I will disagree a little bit or perhaps suggest another reason for my friend Graham. Although I played on hard surfaces and challenging situations, it was not this that differentiated or influenced good players.
From my experience and observations, skillful players became that way by playing against older players in these environments.
The age play was vertical, not horizontal (pure age). Today players play 8 years with 8-year-old, 9-year-old with 9-year-old, etc.
When growing up, I played in pick-up games at 10 years old with and against 14 and 15-year old’s. When I was 12 years old, I was on the field with guys 20 plus and my dad.
In a number of the Latino pick-up games, I see skilled 11-year old’s playing with his older brother and dad, grass or not, and that seems to be more of an influencing factor.
I believe the vertical play in age group allows a younger player to learn in real-time from an older player “institutional knowledge,” as we say in business. In real-time, modeling and mentoring are very meaningful and should not be dismissed as a probable cause for quality in less organized play.
Again, just my observations and formulated opinion.
All the best!