This answer is speculative, but it's a reasonable hypothesis for why we treat the plural as a singular, as the OP has asked.
Merriam-Webster gives this etymological information:
Middle English shameles, plural of schamel vendor's table, footstool, from Old English sceamol stool, from Latin scamillum, diminutive of scamnum stool, bench; perhaps akin to Sanskrit skambha pillar
First Known Use: 15th century
The "shameles" is interesting. A bunch of vendors' tables in a marketplace, all taken as a group, would certainly be a colorful representation of a disorganized mess. (I'm thinking of a "flea market", but perhaps with all sorts of parts of animals, in varying stages of decomposition, thrown in the mix, among other things equally not to modern taste.) It doesn't take much to see "the shameles" coming to refer to a big mess, and from there to be thought of as a single big mess.
An analogue would be the change from "the United States are" to "the United States is" after the Civil War.
Interestingly, one French translation of "shambles" is "bazar"; I had a French friend who referred to her two small children's playroom as "le bazar".