Giving links to closed sites, where you cannot sign up, is not "these are available everywhere"... If they're available, they're available without signing in, buying or otherwise giving any information. For instance, plenty of good models on several of the 3D-sites, but most of them require an account, and some even a subscription (of varying cost) - which may mean that there is plenty of free or semi-free content, but you'll still need to sign up for it, which many people don't wanna do. Having an account here, and getting to download the same items here, are for many a good thing.