kohlrak: There's also the issue of constraints of external engines to developers, too. What if certain engines end up requiring DRM, for example, since some games might allow to omcuh modding or something like that thus game makers can pirate a game and sell a total conversion mod as a whole new game?
SpikedWallMan: That's also something to consider. If the "costs" of the things that you mentioned outweigh the costs of developing and maintaining something custom in-house, then a company may choose to go with developing their own engine. It's just a matter of what makes the most sense to the game developer and what aspects of the game development process they want to prioritize from a business standpoint.
Which is why you see companies doing it. The main thing with The Witcher, for example, has been the storyline, lore, etc. The actual gameplay mechanics always seem to be the biggest points of criticism for the series, yet it sells really well. By using UE they can still do that and maybe get new mechanics. Meanwhile, they're not trying to deal with the CP2077 monstrosity.
A company like SE, however, has alot of different games with similar gameplay styles, so making all their newer games on the Final Fantasy 15 engine makes alot of sense (and the story of it's creation involves alot of lies inside the company that Tabata not only fixed but took the blame for). Capcom's in a similar boat with RE and Monster Hunter (big creatures on uneven terrain).
Back to a company like CDPR: they're not popping out multiple games at the same frequency. We have 4 main games and a couple spinoffs, and they've been around for a while. It's just not viable for them, because they'd be retooling their engine with the next game they make.
nightcraw1er.488: Yeah, I mean how silly to imagine a couple of guys in a shed building their own computer system…
Orkhepaj: yeah not living 40 years in the past does that a lot
Making your own console is indeed pretty practical at this point. The problem is, you'll rely on open source projects to have games on it. That said, it's a bit of a thing right now, believe it or not.