My two cents worth based on decades of talking to design engineers from many companies while attending AES conventions as a mastering engineer. With the exception of boutique and hi-end manufacturers, products are designed to meet a price point, so a final compromise has to be reached between build/sound quality and features. I think most mid-level and below manufacturers realize that 90%+ of their customer base don’t even understand the basics of filters, much less getting into QS and hinge points, slopes etc. So the price point is set, you’ve designed the piece as good as it can be to meet that, now will YOU decide to compromise on something else in order to add features that 10% or less of your base even understands? No. A good manufacturer would first make the power section as robust and good sounding as possible, and then design a quality bare-bones preamp to match it and still meet the price point. What normally happens is a compromise between those two.
The other way to do it is to design the best power amp and preamp possible, with channel switching and full featured filter sections as you describe, and then price it accordingly. In which case you have a piece that 5% or less of the market can afford or understand.
My final opinion, based on being a mastering engineer and electronics engineer, is the last thing you want is to introduce crappy filtering into your signal chain. You do not want 2 cent filters just to have them, which is what would happen on affordable and budget amps if elaborate filtering was included. This is why God created outboard eqs and preamps.
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