A true space age will involve many ships that never enter atmosphere or land.
A scramjet power first stage for example could overcome the tyranny of the rocket equation (at least on earth).
The issue is orbital velocity is an absolutely bonkers high velocity. Nothing can even come close to that velocity in the atmosphere. The SR-71, fastest air-breathing aircraft ever flow topped out at around 1/9th of orbital velocity. Yes scramjets could certainly beat that speed, once they are developed but even if that doubles the speed of a SR-71 (which already was pushing the limits of heating) then you are still only doing 1/4th of the speed you need for orbit.
So you have a bit of a catch-22 situation - You can have plenty of oxygen for your engines all around you, or you can have the speed you need for orbit - But not both at the same time. Yes, a scramjet can reduce the amount of oxidizer required (by a lot) but to do this you need the extra weight of wings and everything else you need for proper aerodynamic flight.
Some of the newer missiles like the BrahMos are designed like this and have a ramjet powered second stage to save on oxidizer. It doesn't reach anywhere near orbital speed, but with this design, the shock cone takes the majority of the heating vs. the SR-71 where there were many exposed parts and materials.
Or even better - keep the current setup but have a ring of air breathing engines in the booster that work as long as they can.
There's a lot of stuff some extra engineering can do, given enough time and resources. For now, SpaceX is going for the biggest bang for the buck, and they have a very healthy aversion of complicated solutions. But in time, adding a few air breathing engines may become simple enough to be worth it.
At an abstract level, it's the same issue as with aerospikes, the idea has too many caveats, too much expense and not enough benefit to justify the relatively small improvements in efficiency.
https://space.stackexchange.com/questions/6256/why-were-jet-...
The summary is that there isn't enough oxygen at the range of altitudes needed.