Some of them are offered at better public high schools still. It often depends on the size of the school. My HS offered them, but it was a "science and technology" school with 3,000 students (largest in MD). Alot of how far a student can go depends on the middle school, though. My middle school only offered algebra, so I was "trapped" in how far I could go. I had to take Geometery, Alg-II/Trig, and Pre-Cal my first three years of HS. My senior year I opted for Stat instead of Calc (Both were AP classes) There was one girl at my hs who took geometery in middle school, then took math classes over one or two summers. She ended up taking Chaos Theory or something similar her senior year at Maryland-College Park. With Chem, my HS offered Chem, AP Chem, and Organic. I took Chem and AP Chem, and that earned me credit for Chem 103/113 at Maryland. I regret not taking more AP classes now.
Smaller schools simply don't have enough students willing to take advanced classes to offer them. Another hs (actually, my neighborhood hs), a mile away, sat mostly empty with 800 students and a poor record, despite being in a not-so-bad neighborhood. (Most of the good students went to my hs on transfer.)