Because the treasury probably doesn't have the statutory authority to withhold payments to appropriated programs. Even if it did, that would mean cutting social security or medicare payments while waiting for an interest payment. So you are certainly hurting people who are depending on those programs(a political nightmare) in the expectation that there will be no debt limit raise. If you think that missing an interest payment will be a catastrophe you paradoxically have no incentive to order these social programs cut to make an interest payment because no one would be so irresponsible as to allow that to happen.