There are several large, quality mods out there, but most of them will not start appearing until sometime after gamesas has released the last patch and DLC for Skyrim and have likely announced their next project (likely to be FO4.)
It's for the afore-mentioned reasons plus one: modding is, at it's heart, a hobby. gamesas made it incredibly easy for us, but nobody can do it full-time unless they don't need to work. This limits modding to after work and the weekends, at most 72 hours of work per week (assuming no breaks for food, socializing, etc. and only stopping for 8 hours of sleep.) Not to mention it is largely a single person or 5-10 on major projects. gamesas by comparison, has a 40-hour work week (which can expand to 80 and still leave weekends free) plus about 100 personnel all working on the same project. I'm assuming that employee number, but it's defantly more than most modding groups. So gamesas is capable of releasing DLCs faster than most modding teams. Now, quality of said games are of issue (gamesas, replace the QA department, or give it more employees or make 'em do their job, dammit!

) but it's still faster than what Mod Teams can usually do.