My 20F Girlfriend was dishonest to Me 21M about her past why ?

My girlfriend and I were having a fun talk, and then she asked me whether I had ever "made out" with any girl. I said no (it is true) and then I asked her that same thing, and she said it a few times, and then I asked whether she had sex before or not. She said she only had sex once with a guy because she felt horny (this convo happened before we got into a relationship) and in different scenarios she mentioned "I had sex once before" a few times. Once she was feeling bad because people were calling her a bitch. I was supporting her and then asked" why do you care when I already know everything about you?" Then she stated that "my friends are actually surprised because I told you everything". After a few months (now), I went to my girlfriend's room, and she was not there, and I started drinking with her roommate (she was a good friend of mine) and then asked her about my girlfriend. She said" she lied to you about her past". I asked what the past was. She told me that she had sex with four different guys, two times with two guys and once with a guy she liked, and that guy ditched her after having sex with her, but she told me that they only "made out" and I don't know the fourth guy's name because I don't really care, because she really takes good care of me. I clearly notice the things she does for me. I love her a lot, she is my world. I want this relationship at any cost, but my real question is why she lied to me and told me the same lie multiple times even though she knew nothing would change, even if she had told me. I felt bad because I had to learn the truth from someone else.

I wanted her to tell me about her past and accept that she was dishonest to me.