leisure=park has a pretty narrow definition, which differs from what we call "parks" in the US. It implies manicured public space. Many "parks" in the US are better tagged as nature reserves on OSM. 
From the wiki page on leisure=park: 
"This tag is intended for (usually urban) parks with managed greenery, located within settlements and nearly always open to general public [...] They are largely a human-sculpted landscape."
https://wiki.openstreetmap.org/wiki/Tag:leisure%3Dpark