Bang pretty much nailed it...though you can sometimes find the right suitor for a heavily modified car and recoup some of what you put into it. If I remember correctly there's a site out there just for selling modded cars and people have some success there. But to be honest, I wouldn't buy my car in the condition it is in now with the mods I've done to it...no way, no how. Kinda like Bang said, there's a lot of joy in doing your own thing to your car and everyone has their own taste and plans to match that. So thus is why you see the price you do for that car.
It is what it is but if I were you I'd try my best to find a "virgin" STi/WRX...they are out there, usually owned by 50+ year olds that wanted something fun to drive but had no interest in taking it further than stock. Likewise, when you start modding your car after you get it, either keep all the stock parts or just come to peace with the fact that you won't get a return on your investment and may have a hard time selling it when the time comes. For me, I'm going to keep the STi in the family and hopefully give it to one of my kids one day...or it'll end up in the bone yard. One of the two but I'll never sell or trade it...not gonna happen.