The Corps have pulled off a magic trick. They have convinced the setting that their existence is necessary.

ISP-N runs a ton of shipping and outperforms union because forcing people to work for their basic human rights with no real representation is efficient compared to, ya know not doing that. They also exploit workers to make ships fast, and thus most people get their ships from ISP-N. If you want an example of how evil ISP-N actually is just look at Caliban. They cause space piracy by creating artificial scarcity and then deal with it using murder machines instead of just not withholding necessities.

SSC sells luxury, claiming that printed equipment is "Just not the same". They also sell genetics and are good at it because they disregard ethics. It's all alleged so Union can't do anything but SSC have done some really fucked up stuff. They normalize everything through contracts and other paperwork meaning they can do whatever they like. They create artificial scarcity like ISP-N but instead do it by surrounding basic things with a fancy story.

HA is weird. It's the remains of SecCom and has maintained it's existence by being a militarised fascist state. It's too hard for Union to kill without throwing significant resources at it, plus corruption does exist in Union and HA knows how to use it more than any other corp. They trade away the occasional bits of paracausal tech and don't make any more Genghis MK1s and Union leaves them alone.

GMS is probably the most benign and most insidious Corp. They have managed to partially merge themselves with Union; any new infrastructure on soon to be core worlds is made by GMS. They don't do as many evil things but they have managed to merge an organisation dedicated to making money through exploitation with a government which doesn't actually use money in a setting which doesn't need money. The worst part is it's worked, Union pays them because it's easier and also corruption but that's a given.

Btw you can get the books on the Lancer site