Because most antennas don't have weird enough requirements so standard designs work fine. For stuff down here on Earth we deal with a lot of the same problem so

Because most antennas don't have weird enough requirements so standard designs work fine. For stuff down here on Earth we deal with a lot of the same problem so the problem has generally been solved to a reasonable approximation and there's additional constraints like shape and space. Standard designs are also just cheaper because they're made by the thousands and machines exist to make them in massive quantities.