Back when the first synths were concocted by Bob Moog and Don Buchla, you didn't encounter a lot of DC-coupled amps. Those came along a little later, as solid state electronics for power levels typically used in amplification became more common. As a result...plus as a result of the idea that you should be able to tap a CV at any point in a patch where it might be present, including the output...the idea that the output stage of a synthesizer should be DC-coupled was rather typical, and even persists to this day with some module designers. And that's yet another reason I strongly suggest to users that they make use of a proper output module, because these tend to be (but not always!) AC-coupled only. When you have a transformer or other isolation device before that final output point, the potential for DC-caused failures drops to zero...and you also get a bit of iron in the signal path that might sound good if pushed into a bit of saturation, plus that also helps with noise and ground-loop issues. Note that this isnt the result you get by simply using typical attenuators to step the levels down for output...that, in fact, is guaranteed to pass any DC that is present on the input side of the attenuator, just scaled down but still quite capable of causing harm.