It's simple.
And it has nothing to do with it being too difficult to hack.
It's all about money, really. Modchips take time and money to develop. Lots of time, and lots of money.
If you historically look at the PSP hacking community, you'll see that there were many modchips planned for the console. What happened then? A week or so after the first modchip was released (it was more or less a dual firmware modchip, anyway) Dark_aleX came out with his custom firmware, IMMEDIATELY removing any need for a modchip.
The team that developed the modchip hoped that they would recoup losses from chip development with massive sales (like what happens with all modchip teams/manufacturers). What happened instead is that their development was used as a tool to develop a chipping-free solution, essentially killing any need for their product. Keep in mind this team is a large team of well-funded incredibly good electrical engineers and software engineers. This killed their business and they lost a lot of money in the process.
Most big-time hackers are afraid of the same thing on the PS3. It would be incredibly expensive to develop, and if history repeats itself, we will have our hands on custom firmware quickly that will lead to the death of the modchip (unless it's necessary to install said firmware).
That's about it, really. Making modchips is a business, and this is a huge risk to their business. They just don't want to get into it.