Because it's not perfectly good. It traps the Heat and won't dissipate properly. If I was to heat this thing up it would take a half an hour for it to cool down. I
That's exactly what its supposed to do, sink the heat. " HEATSINK" By definition the heat from the back of the radio is absorbed like a sponge by this HEATSINK!
I have an old Texas star 250. None of the Texas star amps have fans, just a HEATSINK. I had the same concerns so I put a 4 inch fan on it with the fan blades blowing away from the HEATSINK so the fan is drawing the heat from the HEATSINK and dispersing it. Now I can ratchet jaw for hours and the HEATSINK is barely WARM as opposed to getting HOT with no fan. Not only that, but in minutes of non activity it's literally cold to the touch, That would take any HEATSINK hours to do that!
If you look at those class C splatter box amps, they have fans but practically no HEATSINK compared to a Texas star that's all HEATSINK with no fans.
You seem like a crafty person so why not attach two 2 inch fan and be done with it? Just run the wires to a switchable power source in the radio. This is how I wired the fan on my Texas star so the fan power is controlled by the amps power switch.
This is far cheaper and saner than grinding off parts on your radio to put another HEATSINK that will hardly do anything more than what you already have. If the new HEATSINK doesn't get hot, then that would be cause for concern.
Here is a pic of my Texas star with fan. It's attached with blue plastic drywall anchors that fit perfectly between the fins. Click on pic for the big view.