Nobody said that they are.
I am just pointing out that the people who continually parrot "Make your own group" have pretty close to no idea what they are talking about. Making your own group only works in certain circumstances - basically if you have a key that doesn't suit the affixes of that particular week then nobody is going to bother joining it, since it's very easy to pick and choose the easiest dungeon for that week and only apply to those keys.
Making your own group worked fine when people were spamming M+ nonstop for AP. But at this stage in the expansion people are basically just looking to complete their weekly +15 as smoothly and easy as possible. If you have a 15 key of anything you'll fill the group really fast. But if you've got +13 fortified Arcway or Halls or something you might as well not even bother because nobody is interested in running a long difficult dungeon when you can just apply to whatever Upper Karazhan group shows up on Fortified week.
My advice to OP would be to look for groups with lower keys who say they are "pushing". If you get into that low key and +3 it and do a good job, you will more than likely be invited to stay to do the next key they get as well.