Can O' Beans said:
Simply because most kits do "suck", either with poor performance, or poor quality, sometimes resulting in leaks or both. You can pretty much always get better performance for cheaper when you DIY with knowledge gained from taking the time to read up on the concepts of watercooling.
It also depends on what your goals are. Some better quality kits would be desireable if you have the money to waste, don't care about better performance, and just want something easy to install with matched components.
See the kit I posted above. And there are other stores that sell similar kits. There ARE stores that go out, collect great watercooling parts, and sell them at great prices.
The inoveteck, swiftech, koolance, etc. kits have indeed tested and performed poorly compared to heatercore based parted out systmes, and, once long ago, were the only "kits" available. These systems have given "kits" a bad name.
Key is, TODAY, there are many good, high quality part kits available for watercooling, that are comparable to DIY parted systems.
And no, no matter how much reading is done here is going to enable an incompetent tools person to modify the barbs on an auto heatercore. I DON'T OWN A SAW. I HAVE NEVER EVER USED A SAW. So a simple " just saw the barbs off and weld the barb of your choice" is just not realistic. Complete DIY is just not an option for some folks. Comercial kits have heatercores and pumps that perform just as good as a DIY parts, and don't need further modifications.
The other point I would like to make is the satement "kits are no good" is just not helpful. As I point out above, the statement is just not true. Even if you think it IS true, it doesn't help the noob out. Point out WHY a particular kit is poor. Tell that you think the tubing is too narrow, or is prone to kinks, or WHY the radiator is poor, or that the pump will have a poor flow rate. "Kits are no good" is just not helpful, and leaves a noob frustrated.
If you're going to take the time to post, why not take the time to help someone learn?