SetState not rendering when List are involved

main.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';
import 'dart:async';
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 10;
  int val = 1;
  List<Widget> l1 = [];
  List<int> l2 = [];
  void start() {
    for(var i=0;i<_counter;i++){
      setState((){
        l2.add(val);
        
        l1.add(
          Text(l2[i].toString())
        );
        val++;
      });
    }
    
    Timer.periodic(Duration(milliseconds: 1000),(timer){
      for(var i=0;i<_counter;i++){
      setState((){
        l2[i]++;
        
      });
        print(l2[i]);
    }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Column(
        children:[
          Text("0"),
          ...l1,
        ]
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: start,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}