I read that the monthly benefit amount I choose should not be more than 60% of my monthly earnings at enrollment when combined with any other group or individual insurance I have. Why is this?

The limitation is not unique to the D.C. Long Term Disability Insurance Plan; it is an insurance regulation. It was imposed so people don’t profit from receiving disability benefits.

If you have group disability insurance, the most you can receive from disability benefits is 60% of your monthly earnings at enrollment. If you have individual disability insurance, you can receive up to 70% of your monthly earnings at enrollment.