I guess it just depends on what appeals, or makes the most sense to them. Maybe it helps them feel intellectually superior because it's a counterintuitive concept that kind of blows people's minds. Maybe it relieves them of any feeling of responsibility for their own actions. Maybe thinking of fate as being inevitable and unalterable relieves them of any anxiety over what the future will bring. Maybe what they've heard in dispute of the concept was unpersuasive, poorly presented, or just didn't make sense to them. Maybe what they've heard in support of the concept was persuasive and difficult to argue against.
I've been involved in this debate on this site for awhile. (see my answers, if you are interested.) At times, I see it as a rather fine line; it's difficult to nail down the mind body-connection, but free will has always made the most sense to me.
Some say that there is no such thing as freedom because we can't break the laws of physics. That's ridiculous. Freedom is nothing more than the ability to choose one of at least two options. Even a prisoner in maximum security solitary confinement is free to twiddle their thumbs, clap their hands, or perform a number of other activities. How much sense does it make for the freest person in the freest society on Earth to say, "I'm not free because I can't lift a bus over my head with one arm and throw it to the moon."?
Some say that there is no freedom because, everything that has led up to any given moment in time determines what will happen in the next, and everything that exists and happens at any given moment is determined by laws of physics that are so precise and invariable that only one outcome for every detail of the entire universe is possible. My objection to that is, our understanding of physics is incomplete. I don't think we can reliably assume that there is only one possible future. Some people mention "quantum randomness". I don't pretend to understand that, but how do we know whether or not quantum randomness, or some other aspect of physics, allows for some variability in possible outcomes? How do we know whether or not some aspect of physics, that is not well understood, is the key to, our mind-body connection, our consciousness, and our ability to tip the scales of fate in one direction or another? Also, I think it's arguable that the mind, though it is generated through physical processes, is itself not a physically existing phenomenon. How much does an idea weigh? What is it's volume? What is it's temperature? Physical assessments can't measure something that doesn't exist physically, so why would physical laws constrain it? It's indisputably correct that everything leading up to any given moment determines what will happen next; I see no reason why choices can't be among the determining factors.
Some people say that we don't have a will: that we just feel like we do. We don't make decisions: we just feel like we do. We don't have minds: we just feel like we do. That's more nonsense, if you ask me. It's difficult for me to put it into words. What we are is what we are, and what we do is what we do. Calling it the exercising of one's will, or calling it the playing out of chemical interactions doesn't change what's going on. They're only different names and theories for what's going on, whatever it actually is. There is no difference between desiring something, and just thinking that we desire something. There's no difference between thinking one has made a choice, and having actually made a choice. If options were considered, and one was chosen, then a choice has been made, by some means, and by some entity. Any envisioned option has a chance of being chosen. Saying that a chosen option was inevitable is easy because it's after the fact; it's already been chosen, and it can't be unchosen. That's like calling someone lucky after they win a game of chance. The outcome is not a result of their luck; their luck is a result of the outcome. A choice is not made because it's inevitable; it's inevitable because it was made.
Some say that brain scans show that our decisions are made before we know it, therefore our actions determine (what we mistakenly believe are) our choices rather than our choices determining our actions. I think those brain scans are, just as likely, a consequence of subconscious activity. Some people seem to think of their subconscious minds as something other than themselves. That's an understandable feeling, but in fact, we are our whole minds: an inextricably intertwined combination of our conscious, and subconscious, selves: not two distinct separate minds: not a self and a non-self.
Regarding your comments regarding biology, I don't find the wasted energy angle particularly compelling. It seems to me that you're making assumptions for which there is no basis, namely that generating consciousness would cost extra energy. I personally think of it as more like a side-effect, kind of like a free bonus, or maybe an unavoidable glitch, integral to, and/or resulting from, the complexity of our decision-making processes and abilities.
It comes down to this: if a mind can conceive of options, and if it can desire and choose an option, then free will has been demonstrated. Mindless things can't, conceive of, desire, or choose, options. So to me, to argue against the existence of free will is to argue against the existence of the mind.