Yes, rack bolts are often loose. One reason i see too frequently (or did when i was full time wrenching) was poor rack strut/brace/extension alignment WRT the frame's mounting boss. Many bikes have these bosses at odd angles (think upper seat stay mounts as example) and/or have no true flat surface for the brace/strut to seat against. How many rack installers take the time to bend the, usual, 4 mounting brace ends with what the frame most wants? very few I have worked with do and many of the known home install jobs I see are nearly never tweaked as could be to fit best. In time the not really well seated rack ends will see more bolt loosening, it's hard to get good torque when the last bit is bending rods and strips WRT the frame mounts.

As an aside or tangent we could talk about another hate of mine... bosses that are not truly a part of the frame (Rivnuts). Andy