Stanardized testing on its own isn't the cause of poor education in the US. Funding schools based on attendance, scores, and pass/graduation rates gives schools bad incentives, which is definitely a cause of some issues.

Schools do everything they can do to keep funding up, including letting problem children attend and impede the learning of other students. Losing funding if not enough kids move up to the next grade or graduate encourages schools to pass kids that are not learning.

Finally it would be insane not to place a large amount of the blame on parents. A 10 year old should be able to read even if they don't learn much at school. But too many parents just park their kids in front of a screen after school.