The tongue-in-cheek answer is that they are not designed by SpaceX.
For large passenger airliners, how would that work? Are we supposed to strap LIQUID rockets to them? Precisely balanced around the center of mass? What if one of them fails? What if one of them is ignited when it should not? Who designs those things? How to guarantee structural integrity? Are the attachment points going to be reinforced? What would the FAA say? I could go on and on.
Mind you, there have been planes designed with rocket-assisted take-off in mind. You won't find any designed for landings outside Kerbal Space Program.
They'd need to be liquid fueled because they'd need to be throttled to fulfill all these functions. I am not even asking where the oxygen (or whatever oxidizer) would be stored...