25 years later why is PS still hogging memory?

Well it has got to the stage now where I am looking to bin Photoshop after 25 years of paying for it.

It's an absolute joke that it works perfectly well when it first launches and uses less than 2GB of memory and then an hour after it is working exactly the same but hogging 13GB of memory and causing my other applications to run slow. 

I see this on my Mac too, it allocates 1.75 GB on startup. I think what it does at startup is pre-allocate memory, and that is probably for performance reasons.

This is not an official Adobe explanation  because I don’t work there, but this might be what is going on: One of the things everybody wants to avoid is an application being slow and laggy, and one way to avoid that is to load as many application components as possible into memory in advance. Photoshop probably loads the most commonly used features, fonts, filters, etc. before you start editing, so that when you ask for them, there is as little loading lag as possible.

I also started two other photo applications that have a similar feature set, and they do allocate a lot less memory at startup. So maybe Photoshop could pre-allocate less. Why doesn’t it? One reason might be that it is the oldest, so maybe there are aspects of the 30-year-old software architecture that keep it from loading less until more of the application can be rewritten. But it is also possible that it isn’t necessary to allocate less. There is a popular saying in the software engineering community: “Unused RAM is wasted RAM.” That’s because RAM is both expensive and fast, so it should be maximally used whenever possible, otherwise you paid a lot for GB that are doing nothing. Now, of course running low of RAM is bad when it means other applications are being starved, but as long all running applications are getting what they need, there is no problem using more RAM as long as there is a benefit. (So this doesn’t excuse inefficiency.)

The other observation is that although other photo editors allocate less memory at startup, they aren’t the applications most people choose. Photoshop is. So maybe there is a benefit to the additional RAM allocation that helps make Photoshop the application that people want to use.

This is something you just have to get used to. Raster image editing moves huge amounts of data around, and it all has to go somewhere. It's not Photoshop, it's the sheer mass of data.