(I worked at SpaceX)
Two words: flight heritage.
Successful concepts are very often reused. For example, heat shields, parachute landings, sea landings, air bags for landings on hard land, etc.
Now, let me try to explain this in something similar: cars.
Cars are all pretty much the same. They have a steering wheel, 4 tires, transmission, engine, etc. The parts all do the same thing in different cars, but we don't always use the same engine, or the same tires, or the same frame for every car.
After making cars for so long, why do car companies still have recalls?
New models of cars usually either start from scratch or start based on a different vehicle already in production (new model year). But if you're starting from scratch, you've lost all the "little fixes" that go into making a car good. Like the difference from a new model EV car to a Toyota Corolla that's been in production for basically 10 years has a very different failure/recall rate. After many years of producing the same car, you fix all the little things and get your supply chains working well.
Now back to space ships. It's the same thing. You have made a new capsule and although the concepts are the same, think about all the little things: wiring, plumbing, controls, software. These are all new and basically untested. They lack "flight heritage" (proven working in space).
For the question as why can't we make Apollo ships or Saturn V's anymore, a lot of plans and drawings were lost. Key people making decisions, testing, or even building parts on an assembly line weren't there. Companies making specialized parts folded or went under, or just stopped making those parts.
Sometimes these can be small issues. Like for the Mars Climate Orbiter there was a problem where two different companies thought they were using the same units when they were not. Or when an accelerometer was installed backward on a different ship, making the chute not deploy.
Now compare this to the Soyuz, which is more like a Toyota Corolla of space ships. It's not the fanciest or has the most space or efficiency, but it has a lot of flight heritage and operational history. And from there, you can make small changes relatively safely.
This is true for all companies making space ships. Really it comes down to how well you test things and a good helping of luck on getting it to work the first time, or fixing it quickly.