Regardless of salary numbers, becoming a manager is not a promotion. It is changing career tracks. It just so happens that the lowest rung on that ladder is somewhat reachable from the highest rung of the software ladder.
The problem in many companies is that the positions into which a senior developer may be promoted do not exist. Other positions with greater pay and prestige are on other career tracks. Positions like lead software developer, software architect, technical complexity organizer, software developer supreme (with tomato and sour cream), or historical catastrophe repetition avoider might not be available, or not available to internal hires.
They could exist. Several people here could probably argue a very convincing case for them. But they don't.
And the reason is because certain people still don't see business software resources as anything but costs to be managed down. Just yesterday, a member of my own household wanted to extract an image from a PDF, crop out the whitespace, and post it as a JPEG to a website. This non-technical person could have spent a whole day--or more--figuring this particular problem out. Having a technically knowledgeable person on hand shortened that time considerably. (I could have done it myself in seconds, but I didn't want to assume responsibility for doing that on demand for the rest of my life.)
The sad thing is that I got no credit for the time saved. I instead got grief for not being instantly helpful. I wasted someone else's time, because I didn't anticipate the need, so that SumatraPDF and Paint.NET or GIMP would be installed on their computer already. I wasted someone else's time because I didn't drop what I was doing immediately, so that they wouldn't have to wait for me to finish eating, or, horror of horrors, actually spend a few minutes to try to figure it out on their own.
That's the problem. The whole party complains, "Why didn't you just call on the Eagles again, Gandalf? We could have been done in three days!" And then when Radagast explains about the dearth of giant fish in the magical appropriations budget, no one listens to a word he says.
The people with control of the money don't quite seem to grasp that software is yet another product, among many, where you get what you pay for. No matter how magical it may seem, you have to put more money into it to get more value out of it. People get "stuck" at senior because too many companies have determined that threshold is as much software talent as they need, and that paying for more would be wasteful.