How do you control your Hulk(s) once you've created them?
You ask why people seeking power don't create Hulks to increase their power.
And it seems obvious that having an army of Hulks under one's control would do just that. But you're begging the question. Anything I can think of to control a Hulk would by definition have to be more powerful than a Hulk. If you already have that, what do you need Hulks for?
Maybe you could engineer your Hulks to be dependent upon something over which you exercise exclusive control (such as the Jem'Hadar in the Star Trek universe, who are addicted to Ketracel White). But you'd better be absolutely certain of that control, because your Army of Addicted Hulks is going to be very motivated to get control over the thing that controls them. Whom do you trust to guard access to your "White"? How can you be sure he won't use that to rule your AoAH?
Maybe you can do surgery on your Hulk Army recruits' brains before you irradiate them, and add some kind of loyalty chip that lets you directly control some pleasure/pain responses, and you can condition them to obey your commands implicitly. But (at the risk of repeating myself) if you can do that, why not use your control technology to control more people and skip the whole Hulk thing?
And if you don't have a way to assure that control, all you'll succeed in creating is something very powerful that you don't control. And that certainly seems like it would decrease the power of its creator. I assume anyone smart enough to actually be able to create an Army of Hulks is also smart enough to think this through.
In universe, we have plenty of examples of how hard a Hulk is to control. Assume anyone thinking of creating one is half as familiar with those examples as you are, and you have your answer.