Though it's really odd that it isn't, and I'm not sure why it isn't.
It apparently was a word for a long time, though. The OED records uses from 888 AD all the way up to 1859. Most of the senses are marked obsolete ("In a fixed or steady manner"; "Firmly, unwaveringly, steadfastly; with confidence"; "Without intermission or cessation"; "Closely, securely"), but the "quickly" sense is marked "now rare".
It makes me wonder why it fell out of use in modern times.