How is it their job to lie to me?

When trying to obtain evidence, investigators or regular officers will make frequent recourse to lies and intimidation to get you to admit to things that you may or may not have done. For example, "If you don't tell us where you were that day, CPS will take your kids away" or "Look, if you just admit what you did, we can let you go" or "We've already detained your wife/brother/mother/father and they've fessed up; just yadayada."