I think by far most are mixtures.
A major factor is that most of us do not have the money to just go out and buy our dream system. We must spend hundreds, and eventually thousands of hours learning and evaluating. We want the best sound for our buck. Technology changes between upgrades.
Also, another major factor is that In the beginning, the 1980’s and earlier, typically there were few high end designers and they would do one thing well. Like Pass… amps… then over time added a preamp. But it was not the cream of the crop. Audio Research did about the best preamp. Speakers a whole different bag of manufacturers.
So, basically no company offered the very best of all components. Then there is the fact that everyone has different preferences in sound. So, buying all the same just wasn’t realistic.
But in the last forty years some companies have prospered and moved from a single engineer / founder to professional high end audio engineering departments. Audio Research and MacIntosh (there are others) come to mind. These companies produce best of class sound from all their components and have great synergy between their pieces. You must like their house sound. Both ARC and MacIntosh are very popular and well respected. But for instance I love the sound of ARC and do not like MacIntosh.
You can see my all ARC system under my ID. It has taken me the last forty years of learning and saving to afford such a system. But no question, that for me, the same brand works tremendously towards better sound. I have owned and heard hundreds of components over the decades. And as I assembled this one I heard the value of synergy between the components. By far my best system.