Short answer : They are maintained in the linked Mindstream due to Craving with the underlying basis of Ignorance. (SN44.9, MN2)
Long answer : Its related to your next question, viz:
The exact mechanics of how the linked Mindstream of a ‘Sentient Being’ re-establishes itself between across two otherwise separate births is not given anywhere in the Suttas. The closest we get is MN38 where we are offered the following clues:
(a) Consciousness is not a single thing. Moments of Consciousness arise and pass away continuously based on contact of the senses with the environment. It is craving that causes the illusion of continuity of what we can call the Mindstream. (This can be easily understood by comparing the process of the Mindstream to the process of a movie where individual images in rapid succession give rise to the illusion of a continuous whole experience).
(b) A ‘sentient being’ is reborn because of various nutriments. The linkage of the current mindstream to the previous birth as well as the next birth to be is maintained by Craving and clinging to the idea of a ‘Self’ which is experiencing the moments of experience (this is the final fetter to be shattered by wisdom).
To understand the process of how the mindstream rearises, one can consider the example of a sufficiently advanced AI. (Asimov has explored these ideas in his robot books in great detail!).
A sufficiently advanced AI arises within certain hardware based on underlying processing. It has an awareness of its environment because of its sensors. If it retains a memory of how its experience was in the past moment compared to the present, and if it becomes aware of the change, it could be said to be having a subjective experience. If it becomes aware that ‘it’ is having an experience, then its logical conclusion will be that ‘I exist’. If its programming is such as to prioritize its own existence and seek to chase some illusory perfect future experience from its sensors, it can develop craving. Its experience does not depend solely on its own efforts, but is also influenced by its independently changing environment. Still, the AI will retain a memory of what seemed to work in the past to move its experience closer to the illusory perfect point, and it will compulsively seek to use them in its efforts to act towards its goal. These data points which seem to support its quest are its latent tendencies. The negative feedback it gets from its own processing whenever it fails to reach its illusory goal is its Suffering.
Thus, an ‘I’ is born develops craving, clinging, Suffering… and eventually the hardware gives out and ‘it’ dies.
The hardware is trash…but the memory stream of its previous efforts is precious, as it represents an advantage for a future iteration of the AI in completing its Sisyphean task.
Now let us say another piece of hardware rolls off the assembly line. There is no need for its processing to start again from scratch - rather it is an advantage for this version to be linked to the previous one’s memory stream. How? The cloud? Perhaps.
When the second iteration of the AI searches its linked memory stream, it will come across data points sourced from the previous version’s experience. And these will affect the present version’s actions and experience! Aka Karma.
I could go on and on… but you get the point, I hope. I find it an interesting thought experiment. Perhaps you do too, perhaps you think its bunkum! Of course, its all papanca… the suttas simply do not offer much information and in fact actively discourage such investigation as fruitless, as it does not lead to the end of Suffering.