Probably backwards compatibility with some ancient webserver from the dawn of HTTP which became frozen into the protocol forever and everyone who proposes fixes

Probably backwards compatibility with some ancient webserver from the dawn of HTTP which became frozen into the protocol forever and everyone who proposes fixes that runs into some grey hair who is worried about xkcd-spacebar'ing someone out there, even though its probably no longer relevant, but standards bodies being what they are it is difficult to accept any risk.