They are expensive. Used ones have limited warranty. They are not portable. They consume lots of power. Most of the time they are used for typing, reading, and editing and, therefore, are underutilized.
Computer labs are open only during school hours.
Labs are noisy and busy. Kids are distracted by other applications on computers instead of concentrating on their assignment. Wheel the lab into the classrooms or let the students take the laptops home, to the library, or to field-trips. The cart has a security lock.