First of all - it may sound generic, but it's true that all children are different and have unique learning styles and speeds. And though there is some value in playing and having fun, many parents often see kindergarten as the time to start getting serious about a child's education.
Some parents we've talked to, like the one in this article, noticed that the earlier children begin school, the more independent, outgoing, and curious they seem. They can also develop healthy work habits early on and form a strong base in the core skills of reading, writing, and math, which you can learn more about here.
Montessori schools are also quite well-known for their early years program, Casa. The Montessori philosophy encourages young students to guide their own education using certain tools and texts, which makes sure they're learning while playing. And many parents have appreciated the transition between preschool and more formal classroom environments.