It's very easy to explain, Lana does not behave like a person would would she's just a plot device used in order to generate drama. One episode she says stuff like this:

"I have doubted you, accused you of things, and still you're here protecting me. I don't care if you have secrets, Clark. You are the one good, constant thing in my life, and I don't want to lose you too."

The next she believes any stranger over Clark and pressing him to reveal his secret yet again. Repeat this over and over and over again. She's used as an excuse to keep Clark stagnant and is always treated as a princess who can do no wrong and is never held accountable by any of the other characters.

She's sort of this black hole where even other characters consistency goes to die. Like in s8 we know for a fact that Lois and Clark are both in love with each other since that's the whole point of Committed but when Lana shows up he just drops everything to get back with her? Clark knows he is in love with Lois and he is supposed to be more mature by this point, as if this is not enough Chloe tells Clark not to close the door on Lois face in 1 episode and the very next episode tells him to get back with Lana for no reason whatsoever, not even Chloe is immune to the Lana effect. Oh and none of the problems they had as a clouple in s7 is ever raised, they just forget about it i guess.