Pop artists have generally had more outside writers doing their stuff, at least through what I've observed over the years. What was really nice about hard rock and metal acts was that they, for the most part, particularly while they were establishing themselves, they wrote it all themselves.
Take Aerosmith. Throughout the 70s, they wrote that stuff themselves. And then in the mid-80s, they started employing outside writers to help polish up their stuff and make it more appealing to a broad audience. It worked. Bon Jovi, sorta the same deal. But they have done it ever since their third album, Slippery When Wet (and the first one was pretty much a Jon solo record).
But metal bands, are at least heavy bands, rarely did it. You might see a co-write with someone here and there, but generally speaking at least for me personally, the fact those bands wrote their own stuff was why I gravitated to them.
Here's a big disappointment. Take Revolution Saints, the band fronted by Deen Castronovo, who put out a couple records on Frontiers. I bought the first one, and then saw the band didn't write the songs. I got rid of it (the physical version). It just takes all the steam out of it for me.