Lures and Incense don’t attract the pokemon on the map, they cause random pokemon to spawn, which is why they stack with one another, and those of other players, and which is why pokemon that aren’t even supposed to be in the area will spawn in them and why not everyone will get the same ones.
Pokemon on the trackers/on the map are there for everyone… if 15 people are all close, all 15 will have a random cp version of that Pokemon spawn… Lures attract identical pokemon (if 15 people are at that pokestop, and a Pinsir spawns to the lure, and one player sees that Pinsir has 187 CP, the other 14 players will all see a 187 CP Pinsir as well), Incense attracts pokemon that can be exclusive to the incense user (I can even give a real example of this, as I and a dozen other players were at a pokestop that my friend hadlured, I put on my incense, and I had a 862 Magmar spawn, while no one else could encounter the Magmar at all)
So to answer your question, because trackers help you find the specific pokemon you want/need, but those pokemon have a limited time they will be there. Lures attract random pokemon and ignore the regions for what will spawn and create a specific version (no stat variability from player to player) of the spawned pokemon. Incense attracts random pokemon, but attracts them only to the player with it. Both Incense and Lure have rarity of pokemon increase as timer ticks down (meaning out of the 30 minutes, the first 15 minutes will be low rarity, cheap, probably easy to find pokemon: Pidgey, Rattata, Spearow, Paras, Caterpie, Weedle, Zubat, Doduo, Poliwag, Venonat, Magikarp, Psyduck, Goldeen, Staryu. The following 10 minutes will have moderate rarity pokemon, Starters, Pinsir, Evolved versions of the previous list, Jigglypuff, Eevee, Scyther, Tauros, Mankey, Machop, Rhyhorn, Diglet, Meowth, Ponyta, Bellsprout, Oddish, Abra, Slowpoke, Krabby, Exeggcute, Drowsy, Gastly, Shellder, Horsea, Magnemite, maybe Vulpix or Growlithe. The final 5 minutes is where you can get spawns and stealth spawns of really rare stuff: Hitmonchan, Hitmonlee, Magmar, Chancey, Electabuzz, Onix, Kabuto, Omanyte, Jynx, and evolved versions of the previous lists.