I mean some of them are, but there's no sampler that will painlessly play modern sample packs, no usable pitch correction, nothing a tenth as good at noise reduction / audio restoration as Izotope RX.
Also, maybe a violin is as good as a guitar, but you can't hand a guitarist a violin and expect them to be fine with that. Or vice versa, if you prefer.
I like, use and pay for Ardour, but I also like realism.
But why is there no Izotope on Linux? People gripe at free software developers for "not providing Izotope", when in reality that software represents years of dedicated R&D, something that most people are not willing to pay for (even on platforms other than Linux). The simple reason why Izotope and Kontakt and Melodyne are not available on Linux is not that free software developers haven't written them, it is that the companies that do write them have chosen not to make them available.
I was very fortunate to start Ardour at a time when I did not need the income. Expecting to see world-class plugins like these show up without the involvement of the companies that did the R&D (and/or defined the proprietary file formats in use) is naive.
FWIW, I have talked to Melodyne in the past about adding support for their non-linear data access API, but they have been "unable to come to a consensus about how we could permit this in an open source project".
So for sure, these kinds of plugins are not available on Linux: because their developers have chosen that.
Edit: To be clear that isn't intended as a gripe at Audacity, just at the misrepresentation of Audacity.
The point is this: its up to the user to gain maximum value from their investment. Nobody is going to do that for you.