What limits my intake of other foods is the fact that everything takes preparation, and I have about 0 energy on a good day. I know how to make simple pasta, a simple soup, and a tex-mex spread of fresh veggie-packed salsa and sweet potato chili... that takes about 2 hours to make, after going out and shopping for all the very perishable ingredients.
I'd very much like to experiment a bit more and expand my horizons, but the problem with doing that is that I have no confidence in my ability to make something good on the first try, and fucking up dinner is a massive hit to however much I'm managing to hold up that day. Wasting food (either by having it go bad, or using it to make something inedible) is also a serious killer for me.
Having something that takes reasonably little time to make (my stove is AN AWFUL OLD BASTARD THAT SHOULD'VE BEEN PUT OUT OF ITS MISERY DECADES AGO, so everything takes a bit longer than "fast"), is piss-simple and that I know I can make, and that is made primarily from ingredients with a decent shelf life so I don't have to get bogged down in trying to meal plan or go shopping every day... Yeah. It's a survival mechanism.
I'm trying, but it takes time. I've only just recently gotten the soup down, which at least lets me get some broccoli into the mix.
Also, fish is expensive as balls. Well, decent fish is expensive as balls. If you want some sickly gelatinous salmon-lite fillets, those you can sometimes get for a little less money... But I don't have a grill, and baked salmon offends me on a personal level even with not-awful raw materials. But my oven's a tiny combination microwave/conventional with an instruction manual in Italian anyways, so I don't think I'd even be able to find the settings to make... that.