There are many reasons; the person may simply be naive[1] or gullible [2]
They may also have autism spectrum[3] disorder (sometimes called Asperger’s) and simply be unable to process facial expression, tone of voice, and other nonverbal cues that would be a tip-off to a neurotypical person that they were being ‘played,’ lied-to, or taken advantage of.
It does take some time for a neurotypical person to have enough experiences with other people to learn the ways of discernment—we aren’t born untrusting or suspicious, and most of us would like to be able to trust family, friends, and people we like to speak truthfully and behave fairly. Unfortunately, the way we learn distrust (discernment) is to be tricked and taken advantage of, and that’s just the human condition.
Footnotes
They were monetized but I am giving them away for free here.
If you feel inclined to support my writing, here's my paypal
And if you prefer to pay it forward, I recommend Safe Place as an excellent place to support.