If Ship X goes at 0.75c relative to the sun, and ship Y goes at 0.75c relative to ship X (in the same direction), then ship Y goes at 0.96c relative to the sun, not 1.5c.
It just DOES. Einstein worked this out to be consistent with what actually happens. Don’t blame me or him, blame the universe.
In fact, if rocket sled A goes at 500mph and B at 300mph on top of it, then B will go slightly less than 800mph, but immeasurably so.
[hijack]Of course, a rocket sled is a bad example, because B’s speed will be limited by air friction, and probably won’t be able to resist being blown off the back of A. If it was in a closed container it’d be ok, but then the air would be going at the same speed. If B used a form of propulsion that propelled it off A somehow (say, really grippy 300 mph wheels, ok not realistic) it would break the sound barrier.[/hijack]