What would make a parent lie about their childhood to their own children? [I Am Depressed]

Ok, I have NO idea about your mother's situation. Here's some hypothetical.
1. Sometimes people go through things that they want to protect their children from. When your children are really young, you certainly don't confront them with traumatic dysfunctional family issues. It's not necessarily lying, it's simply treating it as water under the bridge.
2. Some people's childhood is so traumatic that they have been unable to deal with it. Everyone has coping mechanisms and most of us have a few really bad ones. If your mother's coping mechanism is to push it aside, then she's doing the best she can to protect you from her own pain. A lot of what good parents do is not pass on every emotional struggle they have to their children - maybe she's overdone that a little.
3. Maybe after 30 years she's struggling with the change of having adult children and hasn't found any space that makes sense to say my dad was a drunken this or whatever... It's harder than you think if there is trauma involved. Because adult children can be harsh. They often don't really want their parent as an equal, even in their thirties.