I feel that people tend to (mis)apply the "polysexual but monoamorous" label to people who don't appear to want to commit to them or anyone else. Commit by their standards, anyway. The truth is, like others have said, someone might not be in the right place to develop (additional) relationships with the features of traditional romantic commitment but it doesn't mean they won't ever be and it might just mean they don't feel they are compatible with you for that type of relationship.
Poly doesn't mean that every single relationship you choose to pursue involves or even had the potential for involving romantic love.
I do agree, however, that some people who genuinely want polysexuality with monoamory feel compelled to adopt the polyamory label due sex negative stigma. The apparent focus in polyamory on love and it not being simply about wanting (dirty/sinful/shallow) sex with loads of people purifies non monogamous desires for some people so they stray to this relationship style when NSA swinging would possible be a better fit for them.