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Non executive hires typically do not have enough personal uniqueness to the employer to have much power to do this kind of thing. The likely result is that the employer with an overreaching employee agreement will just move on to a more docile or naive candidate. The job seeker usually needs the job more than the employer needs the job seeker.
Executives also may have more personalized agreements to begin with, such as golden parachutes that do not apply to other employees, so there are more things that they may want to negotiate.