If you replace "developer" with "general contractor" and "writing code" with "constructing a building", do you believe your argument holds up? How about "doctor" and "practicing medicine"? "Attorney" and "practicing law"? "Salesperson" and "selling effectively"? I could go on.

While employed, a person necessarily learns skills in order to perform their duties; as an employer, it would be impossible to derive value from the worker without training and knowledge transfer. Your argument is that it should be a two-way street until employment is terminated, then it becomes a one-way restriction against the employee. That's not fair to workers and is extremely biased toward the employer.