Insecurity is the "benefit of the doubt" reason. Unfortunately, far too often, people are dishonest because they want something, and they know the truth will impede them from getting it.
For example, one approach (of which I do not approve) is to tell someone you're poly only after they've started to fall for you, when you reach the point in the courtship where you start to talk about "having a relationship" instead of just "dating." By then, they know how well you get along and how they're starting to have feelings for you. So maybe poly isn't their first choice, and they wouldn't have let it get this far if they'd known ahead of time, but now they're willing to risk it, because you're "so wonderful."
Now in my opinion, this is nothing short of bait and switch. It's totally not cool. But, for some people, it gets them what they want, at least temporarily.
Some people also thrive on drama. I don't get it personally, I hate being involved in drama. But some people just aren't satisfied unless they're causing trouble for someone else.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker