Not everyone is motivated enough to build it themselves if they have the means to buy a prebuilt.
Having said that, an exercise bike was sort of my gateway to hobby cycling and I ended up investing in a smart trainer and a used road bike. Once you set it up and find an app you prefer, you can’t go back. Just as you can’t go back to buying a prebuilt PC if you’ve built one in the past.
A little bit of research goes a long way, but some people would rather not and just focus on what’s important: getting fit. Setting up a smart trainer, although extremely simple, wasn’t obvious when I first got into it. I wasn’t sure which parts to choose and why.