Having both a 1st and 2nd generation phone, I can say simply because I have never felt a need to do so.
My experimental hacking days are for the most part behind me. I am not really interested in turning my iPhone into a guinea pig for 3 frat brothers in a barn. I actually depend upon it to help me run my business, and having something untoward happen that required my time and attention to revert at an inopportune time simply was never worth whatever program someone might have been offering out there.
Short answer my phone is more important to me as a tool that I would not risk/sacrifice it for the chance to make some homemade icons, ringtones and wallpapers.
If I was at a time in my life where my iPhone was just a toy then I would probably have no problems jailbreaking it and playing around with all the programs available... but again.. not a toy.
I also completely agree. I used to build my own pc's, hack XBoxes (soldering in Xecuter chips) etc.
Now I just want to use the stuff to get other things done, not turn the tool into a hobby. So now I just use Mac's and AppleTV's in place of the above and they just do the job out of the box. So does the iPhone.
I actually find the iPhone does what I want and does it beautifully. I don't want the other 'features' offered by hacking, so why would I bother?