why the view is up when keyboard show?

i do this code:

struct TextView: View {
   
  @ObservedObject var service = Service()
   
  @State private var text: String = ""
  @FocusState var isInputActive: Bool

  var body: some View {
    ZStack{
       
            Color(red: 242 / 255, green: 242 / 255, blue: 247 / 255)
        .edgesIgnoringSafeArea(.top)
       
      VStack {
        Text("QRCode Text")
          .multilineTextAlignment(.center)
          .font(.system(size: 30, weight: .heavy, design: .default))
          .padding(.top, 16.0)

         
        TextField("Enter Text", text: $text)
          .textFieldStyle(RoundedBorderTextFieldStyle())
          .border(Color.white)
          .fixedSize(horizontal: false, vertical: true)
          .multilineTextAlignment(.center)
          .padding()
          .frame(width: 350, height: 80)
          .background(Rectangle().fill(Color.white).shadow(radius: 20))
          .cornerRadius(30)
          .frame(height: 100)
          .focused($isInputActive)
          .toolbar {
                    ToolbarItemGroup(placement: .keyboard) {
                      Spacer()

                      Button("Done") {
                        isInputActive = false
                      }
                    }
                  }

        Image(uiImage: service.generateQRCode(from: "\(text)"))
          .interpolation(.none)
          .resizable()
          .padding(2.0)
          .scaledToFill()

          .frame(width: 250, height: 250)

         
          .fixedSize(horizontal: false, vertical: true)
          .multilineTextAlignment(.center)
         
          .frame(width: 350, height: 350)
          .background(Rectangle().fill(Color.white).shadow(radius: 20))
          .cornerRadius(30)

        Button("Save QRCode") {
          print("save")
        }
        .padding(.vertical, 10.0)
        .padding(.horizontal, 100.0)
        .background(Color.blue)
        .cornerRadius(20)
       
        .foregroundColor(Color.white)
        .frame(height: 100)
         
        Spacer()
      }
    }
  }
}

and when i click on textfield and keyboard up, it up all view, why?