nun

There's a couple of things going on and as panniers on racks are generally a long way from the center of mass of the bike it's better to mount them down low close to the contact point with the ground. Putting a saddlebag tight up against the seatpost is a way to keep the center of mass very close to the rider and minimize effects on the handling of the bike. It's also nice that there's no issue with keeping the weight balanced between two panniers. It's a set up that's been used for many decades and has been adopted widely for bikepacking, although there is a tendency to add top tube and frame bags too. The handlebar bag position isn't ideal, but again the weight is quite low at about 6 lbs loaded and the convenience is worth it. Having everything in just two bags that don't mount to the frame makes the set up very versatile so that almost any bike can be used to tour. It also makes it easy to lift the loaded bike over obstacles and up and down stairs.

But to answer the OP's question, if a bike has the attachment points be it carbon or steel you can use racks and panniers, but make sure the geometry is right as well. As an example you can fit a rear rack to a carbon Specialized Diverge and hang some panniers on there, but make sure you don't get any heal strike.