Slavery is indefensible because humans aren't artificial constructs built on an assembly line. Subjecting machines to similar conditions, even if they are intelligent, is okay because at the end of the day they are still built to be our tools.
I would even argue that non-sapient animal life is more deserving of rights and protections than a sapient machine.
"Made on purpose to serve" isn't a good qualifier, honestly. If I get my wife pregnant while intending to make the offspring a slave, then that kid was only built to be my tool. We disallow slavery because of sapience, because we value the human mind over all other things.
There's no metaphysical difference between intentionally creating a child and intentionally creating a robot, the 'intent to create' is the same. The only difference is sapience.
Fun fact: a variant of your argument was used to justify slavery. "Those kids wouldn't be here if we'd not intentionally bred our slave stock". And it's not just the autonomy of the initial slaves that matter. If I were to create a sapient AI using slaves, non-sapient AI, or employees, its moral status would be the same.