I think its about the build quality, the service, the instructors, and the integration. I mean maybe you can get a separate bike with a resistance knob that matches peloton's numbers, and track your workouts with an Apple watch, Fitbit, or something similar, then rig up a tablet to follow along with the instructor, but that's a lot of hassle and while maybe you already have the tablet and fitness tracker, if you bought them specifically you aren't too far off from the price of a bike. I am not sure if you could get the competitive aspect of getting a real time update on the leaderboard as well- which while isn't strictly necessary, it does motivate me at least a bit, and watching it keeps me from getting bored.
I don't know how widely spread this is, but even before the pandemic my company had a gym benefit and it can be applied to Peloton. I didn't know that going in though, I just had a discussion with a coworker after we got it where they told me that.
And yeah, when I was in my younger years when I had more time than money, I might have cobbled together some solution like you are suggesting, but these days, I can pay for the nicely integrated solution that I don't have to debug or worry about.