Because it's more time and effort to make a service that checks spelling (are you accounting for stems? Possessives? Other uses of the single quote such as cont

Because it's more time and effort to make a service that checks spelling (are you accounting for stems? Possessives? Other uses of the single quote such as contractions?) and suggests corrections than to wrap an already-existing service that does the same. I find it highly plausible that the author of the plug-in wrote it to solve their needs and just made it available because hey, it doesn't cost them anything.