If you watch MLB games you will see many different sizes, different deliveries, different angles, etc. That in itself makes it harder to hit!
The secret to having a mid 90s arm, is to be born with one. That said, there are more outstanding pitching instructors now than ever. Problem is there are also a lot more that use the cookie cutter approach for everyone they teach. Truth is, individuals don't all throw exactly the same way. The type of mechanics that work for a John Lester might not work for a Randy Johnson. Furthermore, when you take away the things that are natural you "could" be increasing the risk of injury. Bottom line, there are thousands of those cookie cutter pitchers that can't pitch at the next level.