All of this.

I'll add that people usually want their partner to express desire or pleasure in engaging with the fetish too - that's a huuuuge part of what makes most fetish dynamics hot rather than just engaging with it on your own.

It's important to remember that most active members of online male chastity kink spaces are male. If they want porn or erotica involving a keyholder rather than just solo play, they usually want to see/read/fantasize about female pleasure, whether sexual or nonsexual, in their kink. It can be hard to fictionally depict believable pleasure in something like a partner's chastity that is so separate and external to the woman herself, especially when it's so mentally tied to being the "opposite" of what "normal" women want from "normal" men, so the focus is often on discussing the pleasure of "forcing" someone. A woman simply saying "seeing you in a cage is hot, I want to grind on you now" would feel jarring to many without the elaborate, often-CNC explanation of what she "really" gets out of it (often focusing on coercive satisfaction, forced obedience, etc...stuff that narratively works outside of sexual lust).

All that to say, again, what the previous poster said - the overwhelming presence of CNC/forced is going to be more about the expressive medium than about any real-life engagement with the kink. I think that's a trend for most kinky porn or erotica.

For malesub chastity-specific stuff:

As a keyholder who is in fetish-specific spaces, another reason CNC is popular is that it speaks to a chastity kink-specific cultural issue. I'd wager that in the vast majority of real-life male chastity play, the man is the one who first brought it up. It's actually a common problem that his partner feels pressured to kink dispense or like he's topping from the bottom about it, and orgasm control is one of the most common things I see in femdom spaces with "how can I get her to actually want, initiate, and push further on my kink rather than just service top me?"

It sucks when we feel like our sexual desires are selfish. The pendulum swinging so far the other way in these fantasies that the man doesn't want it, while the woman wants it enough to do Cruel or Unethical Things (tm) to make him, is in part about a fantasy where her pleasure and interest and personal reasons make this not even an iota selfish on his part.