Because these places democratize power. They let you, a normal person, walk through the same doorways as presidents. You can touch the same desks (okay, maybe not touch). You can feel the weight of decisions made in those rooms.
They also contain secret stuff. The Obama library has a digital hologram of the president. It gives you a “personal” tour. It’s like having a ghostfriend who once ran the free world. The White House itself is closed to most people. But any citizen can enter a presidential library for a small fee. It’s your taxpayer-funded VIP pass.