The vast majority are doing it for fun - not too different from buying a racing motor bike...
Motor bike racing is a competitive sport, CCR isn’t (or shouldn’t be!). For me diving isn’t akin to trying to win a race or whatever and I think the racing motor bike analogy actually touches on something else I’ve found interesting though, and it exposes the underlying reason so many amateurs decide to go CCR; ego. CCR is often seen as the ‘pinnacle’ and OC divers perhaps allow themselves to feel like they are missing out somehow.
Just like those who think they’ll never crash their racing bikes (even though plenty better riders have) because they are more careful or more skilful or whatever, we all kid ourselves when we read about CCR fatalities that “it will never happen to me because…” Thing is though, we all make mistakes. Watch the BSAC conference video from this year by Simon Mitchell for more on this (
BSAC Diving Conference 2019 videos of presentations from about 19min). Even the most experienced can get bitten (as you mentioned in the OP).
Some professionals (military, commercial, scientific) choose to use a rebreather as it makes what they do safer and/or more economically viable. Often there are extensive support networks looking after maintenance etc to enable them to do so in as safe a manner as possible. Some amateurs choose to use a rebreather (for various reasons), but I suspect in a lot of cases ego plays a part as they could more safely conduct their diving OC (for a lot less money and a lot more fun).
I became a rebreather diver for work reasons. Like The Chairman I also still have significant fear. For this reason I watch the unit (and myself) like a hawk on every dive and because of this I’m actually having LESS fun. I can’t relax for a minute. I’m so busy making sure I stay alive that I don’t take pictures anymore, I don’t enjoy the wildlife or scenery as much, it takes a long time to prepare the thing (usually the night before) and just as long to breakdown, clean/disinfect and put away again after a days diving. In contrast, my OC set took an incredible amount of abuse for years on end without so much as a whimper. Never washed, never serviced, just pumped up and used then thrown back into the shed until the next weekend. Never let me down once. Try that with CCR and you’ll be another line on the excel list in very short order. The machine needs looking after like it’s the crown jewels and you’ll need to spend a king’s ransom to keep it in top order and on buying running consumables (when compared to OC).
As soon as I’ve done what I need to do, I’ll be swimming back to OC as fast as my little fins will propel me!!
Unless you are regularly undertaking dives that would make sense to use a CCR (cave?), or you ‘need’ one for professional reasons, AND you have the mind-set and finances to support it, then it’s a bit mad (and selfish if you have a family) imho. Just like motor bike racing.