Look at your car insurance, for example. You can buy a policy with a clause that pays for a rental car while yours is in the body shop, or not. It is not reasonable for you to argue "Well, this policy paid for the rental, why won't you?" You probably didn't get the rental car coverage to save money.
When an employer purchases a group contract, they do the same thing. They tailor the contract to balance between the needs and the costs. One employer may buy a policy that has exclusions for pre-existing conditions and another employer may not have that provision.
Social Security functions under different rules that most commercial insurers. Approval by Social Security may influence the decision of a commercial insurer, but you should not presume that approval by Social Security means that your medical condition will also meet the terms of your commercial policy.