You can always force them to show up by making your own map using the map editor in game or the forge. I guess I'm lucky in the sense that I get at least 1 every other map or so.... But they're just eye candy and actually not all that useful and more of a pain in the ass then anything. I'll tell you why.
Huge well radius that slows progress down. Simply trying to drive through it, or build stuff up, the distance one must travel is a pain and time consuming.
Huge well radius that makes it extremely tough to defend. You're forced to consolidate your stuff into pockets making them easy targets and those pockets could end up on opposite sides of the well, meaning anyone who jumps in only faces half, or a third, or a quarter of your defenses and has the time to seriously damage them before any hint of backup arrives. Skilled invaders can outmaneuver all of your defenses and just attack the planet and rob you of the ability to replace destroyed structures even before the real battle begins.
Tough defence makes it a resource hog since things always get destroyed there. You constantly have to rebuild. That's just life there. They take more then they make.
Low population so no tax money.
1 exploration chance instead of 2, so you're down 50% right there.
Very few bonuses the type pulls from (I think it only has 4 or 5 in it's list, could be less).
Resources are just average at best. Really, they should be supporting more then double the resources of the other types since they're so huge, making them somewhat worthwhile to hold. Right now the largest types have 6 resources, the dwarf giants should hold 8 to 10, and the normal should hold 12 to 16... That would sure as hell make it worth the effort to hold onto it...
As it is, right now, I just pass through them and not even worry about it. If an akkan claims it as it lumbers past, I'll put out the extractors, and increase the population cap so it makes some money rather then just sitting idle, but they're really not worth the trouble.
An advantage they do have is they slow the enemy down as well. Gives you time to marshal your forces, but a well defended planet does the same thing, sometimes even wins without the need for fleet support.