I mean, the technical answer is space.
Even if you max out your lair space completely (save I suppose in this instance a few spaces for hatchlings to populate before being auctioned, etc) then you have to sorta choose.
Am I going to keep this pretty fodder? If yes then which perma is going? etc etc
That and... while I guess it's just easy for me personally to ignore it, I don't really care too much if a dragon is 'pretty' or not unless I'm actively looking for them.
I've got a few projects going for dragons that would 100% qualify as fodder in their current state (pre genes/breed since right now I'm just breeding for colors) and have been sold as such. While they might not qualify as beautiful rescue worthy fodder or anything as they are now, with the right genes I'm sure they could. They aren't what I want though so I just toss them out.
The end result will be pretty (to me) but anything that becomes fodder on the way might as well be exalted for 0 treasure for all it's worth to my project lol.
Not to mention just HOW many dragons are on site. You'll see people complaining about the dragon economy all over the place. And that has to do with something that sort of ties into this question.
So there are thousands of users on site. Each breeding their dragons. So besides really rare combos it's not like it's hard to find dragons. Any dragons pretty, ugly, a mixture, etc. The more people there are the more dragons their are. The more dragons there are the LESS they are worth overall.
Some users might remember way back at the beginning of the site (i wasn't there but I've seen plenty of stories about it) when XXX dragons were the literal cream of the crop.
Anyone remember the triple Purble or w/e that dragons name was?
But anyway, back in the day Triples were the most expensive things on the site. They'd go for thousands. You add circuit or iri or shim? Good lord you'd be a millionaire if you had a decent color XXX pair.
Now you can get good XXX dragons for literal pennies. Because there are JUST. SO. MANY of them. Sure you can try to sell dragons for more than fodder and actually succeed in doing it. But like. When you're competing with thousands of other users, most of which are putting their dragons up for fodder prices? All I have to say to that is good luck lol.