OK we could go on like this forever so let's try to make this our last back-and-forth
1.) Fly is physical. Thunderbolt is special. Zapdos' attack is 90, which isn't terrible, but a lot worse than special attack. Zap Cannon has 50% accuracy and is a terrible idea. A good replacement would be Ancient Power to hit Flying types.
2.) This person tagged this in-game, which means they aren't playing competitively. Post game means after the game is already over, so your advice is like suggesting Alakazam to someone who can't trade, or a level 100 pokemon to someone with only 1 gym badge. If you could complete the game with 6 legendaries, of course people would choose that.
3.) What do you have that someone with a ZAPDOS would want? Also nobody uses gen 6 GTS anymore because nobody plays OR/AS or X/Y, so good luck finding a Zapdos on GTS asking for something you have
4.) Last time I checked, ten 6 DOESN'T HAVE BATTLE TREE and either way, the battle mason and battle tree are also post game so Roost is literally 100% useless