Everyone has so many of the same reasons for tying that I do.
It's a way to be connected to the water when you can't actually be there.
It's an artistic/creative outlet.
It's so much more fun to catch fish on something you created than on a pattern someone else created and you purchased.
Better than watching TV...although I will admit I watch a lot of sports, and I love tying while the game is on in the background.
As the OP said, I have so many flies, but I keep tying! One of the reasons is to restock the boxes with the patterns that worked well for me the previous season. Another reason is to experiment with new patterns...whether that be improve on one that maybe had an issue, or solve a problem from situations I may have run across from the previous season.
Kind if a weird one, but I like substituting materials on either commercial patterns, or my own. It might make it easier to tie, or the sub material might be cheaper, or maybe I just like the way it looks on the fly better. But really, it's kind of to show both myself and the collective "they" of the fly tying "pros" that just because they used brown UV Ice Dub (and are likely a hareline pro), doesn't mean that brown Arizona Diamond dub won't work just as well (if not better). There is no "holy grail" material, and don't be afraid to substitute!
Lastly, I take a fly fishing trip every year (sometimes 2!). Winter is slower for me at work, so I have a good amount of time to tie. One of my favorite things is putting a box (or 2, or 3) of flies togethor for those upcoming trips. Thinking about the hatches I will encounter, or the prey that the fish eat. Doing the research and coming up with my own patterns. I tied 3 boxes of ~400 flies for my trip last year. That was the first trip we took where every day but 1 was guided. Not only was the catching great, but I didn't fish a single fly that I didn't tie. That is satisfaction for me.
While we all know tying your own flies will not save you money, I just don't care at this point in my life. I want to fish patterns that the fish don't constantly see. I want to fish patterns that I created, and tied for a specific reason. Not only that, but I am a full blown material hoarder. I love shopping for materials. Hackle, both soft and dry fly, along with obscure feathers, realistic synthetic stuff, and dubbing are my jam. I have enough materials for my great grand kids lifetime. Unfortunately my kids have not caught the bug, and I am trying not to force it upon them.