Message Authentication Code (MAC) are based on symmetric cryptography. Digital Signature are based on asymmetric cryptography.
Using the existing algorithms, symmetric cryptography is by far computationally cheaper than asymmetric.
It means that you can create and verify a MAC for each packet on a network traffic with a reasonable increase of calculation, i.e. without slowing the traffic to much. With digital signature, the cost of signing and verifying each packet will surely create a bottleneck slowing drastically the traffic or needing a huge amount of computational power.