I appreciate and agree with ArvidJ's insights. To expand his first question a bit, I would add c) cost. There was a time when DIY might have been a path to saving money. That is not the case anymore. If one wants a basic, reliable, 200x200x200mm printer, it is hard to beat the various Creality Ender type printers, some available for < $200. If one wants a high-end, larger format (300x300x300 or more) printer, it is hard to beat the higher-end CoreXY machines that are coming on the market. As best I recall, pricing starts at < $700. In both cases, it would be challenging to source the components for less.
That said ... I have never bought a commercial 3d printer. I built my first 3d printer, more or less of the Prusa I3 type, back when there was a little bit of savings for the DIY route - even then, only a little savings in cost, but a big difference in safety and longevity - the low-cost printers at the time were a bit sketchy. For my first printer, I decided to see if I could make a 3d printer as much out of wood as possible - not only the frame, but also the parts that typically would be 3d printed, such as the parts that hold the linear bearings and nuts, that mount the stepper motors, etc. It performed admirably for 10 years or so, and though I have updated it a bit, it still serves as a reliable workhorse for me.
Meanwhile, I have sourced all of the components for a Voron-based build, and look forward to adding that to my arsenal in the next few months. Fortunately, I was able to source everything before tariffs kicked in. Even so, I would guess I've spent about $700 on the components, so this is not motivated by cost savings, but rather by the fun of building something that I can customize exactly the way I want to.