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.
I've found local spellcheckers really unreliable. They never catch obscure or niche words, or grammar. Chrome has an option to ask google for spelling suggestions and it makes it much more reliable.