For me, it's because -- let's face it -- it'll only be available to the extremely wealthy and well-off. It won't be available to the average citizen. This will just further cause our immortal overlords to be able to concentrate power and wealth, even moreso than they're already doing. That's not a good thing. Death is the Great Equalizer (especially if there's a very progressive estate tax), and if it is conquered there's nothing to stop that anymore.
As to why we treat doctors and those who prolong life now differently, well, it's reasonably accessible to everyone, especially in a first world country. Sure, you might not get the cutting-edge treatment the rich and powerful can get, but you can get very good quality treatment and prolong your healthspan for a while based on what we have. And, even if you don't get the treatment the rich and powerful do, they are still going to die one day, so at least there's that.
I see no reason to believe that whoever figures out how to stop aging/death will open it to the public, or that it will ever be accessible to the populace. For me, it's a vanity project of the rich that will only harm the poor, and I think that quite decently fits the definition of megalomaniac.