General MIDI sounds suck.
General MIDI has no sound, just suggested patch numbers for a set of common patch names.
Literally this is true, but I think we all know what they are referring to. The Roland MT-32 pretty much defined the sounds for each of the 128 choices, and so for a company to create a GM soundset that customers thought "sounded good" playing back MIDI files, the new company had to pretty closely match the MT-32. So we ended up with a sea of products with a GM mode that were all trying to sound as close to the MT-32 without being sued...
Perhaps another reason for this perception was the fact that for GM you could only have Chorus and Reverb, so when compared to "regular" sounds that could have insert effects and more diversity in general, the GM sounds were plainer.
Oops, see, I just did it. Because what else are you going to call those 128 sounds? The sounds found in the GM sound bank?