I hate to say this because it is so obvious, but what do you think we did back before factory made knotless tapered leaders hit the market and became commonplace? I learned how to tie my own tapered leaders way back in the very early 60's from my father because there weren't any factory made knotless tapered ones available.
And like Pescaphile, I don't see how connecting a knotted tapered leader to a fly line is any faster or easier than connecting a knotless tapered one. Most of us who have been around fly fishing for many years have been nail knotting a short, heavy butt section (of between 6" to 1') to our fly lines with a loop on the end to which we attach our leaders with another loop. Once you tie a loop (whethter perfection knot, double surgeons, or any of the other loop knots you can use) to your leader butt, it takes no more time to replace a hand-tied, knotted tapered leader or a factory made knotless tapered leader. However, it does take a lot more time to tie the knotted tapered leader than to simply take a factory made knotless tapered one out of the package.