If you're going to build an expendable rocket designing it as a SSTO doesn't really make sense. With a single stage you can't shed the extra weight of the engines and rocket that got you out of the atmosphere and have to have engines that work both in atmosphere and in vacuum.
What I was trying to say is that there are other reasons to not build a single stage to orbit expendable rocket, mostly cost and payload capacity. I agree it'd be easier than a reusable version but if you're not returning the stage it makes way more sense to use the 2+ stage designs that are common today. Being able to shed the weight of the atmospheric stage and switch to a vacuum optimized engine is a big benefit.