Spacecraft that require protection from re-entry are going far, far faster than first stages, so slowing them down using atmospheric friction creates a lot of h

Spacecraft that require protection from re-entry are going far, far faster than first stages, so slowing them down using atmospheric friction creates a lot of heat. The Falcon first stage gets to about 8,000 kph and reaches over 100 km before it starts coming down, not high or fast enough to create the stresses an orbital re-rentry vehicle sees. The other key to reusability is having enough leftover fuel to land, control surfaces and restartable engines. Restartable engines weren't proven to work until the Gemini program in the mid-1960s, if I remember right; I'm sure the Saturn rockets was well into development by that time and the engineering costs to make major changes the lower stages were too great. Sometimes government gets set upon a course and it's very hard to redirect it; that's why innovators like Space-X are so valuable.