Two things can be true at the same time
1. Exploring the concept of Cybertronians being unaware humans are sentient - or, the less scrupulous hiding it in order to make a profit - and adopting humans as βpetsβ is a very well-established trope, and has its place in fanfiction just the same as any other trope. Itβs fun! People like it for a reason!
2. Humans would make the WORST PETS OF ALL TIME, EVER, FULL STOP.
Care requirements are horrible. Complicated. We evolved in harmony with an unutterably delicate and complex ecosystem, and removing us from it without immediately killing us requires three advanced degrees per mech, specialized equipment, and an entire team of scientists, medics, and engineers. If you donβt have the resources of a major xenological park to hand, including the deep pockets of shanix to pay for it all, youβve already failed.
Behavior wise? Humans lose our shit in captivity. We use captivity as punishment for our species. You throw a human in a cage, and they know theyβre in a cage, and it wonβt matter what enrichment you provide. They will become unhinged. They will have one goal: to break out of the cage. They will be angry at you when they break out of the cage.
You are getting the hellish combination of an under-stimulated Border collie, an unpredictable chimpanzee, an isolated Seaworld orca missing their pod, with the spite of a recently shaved cat. When your βpetβ human breaks out, when, not if, the mech βownerβ will wake from recharge to find someone with tiny hands and advanced problem-solving abilities is now living in their walls. They are coming out only with the intention of committing acts of sabotage and violence that would make the DJD think βThatβs a bit much.β They are going to kill you in your recharge.
Besides, humans donβt live long compared to mechs. It would be like mechs keeping a fruit fly as a pet, if the fruit fly had opposable thumbs, a shiv made out of scrap metal, and a personal vendetta against the mech for putting them in a cage.
βBut humans are domesticated, they even domesticated themselves! They love to play and follow routines and respond to positive stimuli! Theyβre so cute when they sleep! They pack bond easily!β
Thatβs Unicron talking. Do not listen.