Since this question comes up pretty much every year, and the quick answers aren’t all that convincing to those who don’t follow cycling, let me answer in a bit more detail.
The Tour de France is not only about who finishes with the lowest aggregate time (aka the GC, general classification). Stage victories are also extremely desirable in their own right, and none more so than winning on the Champs d’Elysees. In addition to that, there are the subsidiary competitions for the Green Jersey (points classification, where riders are awarded points for high placings in both stages and intermediary sprint points) and the Polkadot Jersey (king of the mountains, where riders are awarded points for high placings going across the tops of climbs).
The last stage into Paris is a very flat course. With no hills in sight, it is next to impossible for any single team to drive fast enough to split the pelaton. On the flat, every single rider in the Tour barring injury or illness can keep up with the fastest riders when they can stay out of the wind at the back. Moreover, the pelaton, when driven by one or more teams that really care, can always ride faster than any reasonably sized breakaway.
Now, at this point in the Tour, the GC is wrapped up. Somebody is in the lead, probably by a minute or more. Most teams don’t have anyone within 5 minutes of the lead. But that doesn’t mean that nothing is at stake in the final stage. The stage victory in Paris is one of the most highly sought-after prizes of the year for the sprint specialists, and very often the Green Jersey competition rides on the outcome (this year it doesn’t - Sagan sewed it up in the intermediate stages between the Pyrenees and the Alps with some brilliant attacking riding.) So, the teams of any sprinters that think they have a shot at victory will do everything they can to bring their men to the line without any pesky breakaway out in front. This year, that would include at the least, Lotto Soudal (Greipel), Etixx Quickstep (Cavendish), Giant Alpecin (Degenkolb), and Katusha (Kristoff). Tinkoff Saxo might also go all in for Sagan, who despite contesting every sprint isn’t really a pure sprinter. Between these teams, you have 30+ riders who will be extremely intent on crushing anyone trying to escape off the front.
It isn’t completely impossible to win in a breakaway. See, for example, Alexander Vinokourov’s incredible ride in 2005 - but he only won by a few seconds, and was doped to the gills to boot. He was also the greatest attacking rider of the last two decades. Even worse, Quintana is singularly unsuited to winning in this way. He doesn’t have devastating speed on the flat. He’s only faster than everyone else on 11% gradients. To win in a break on the Champs d’Elysees you need a rider like Tony Martin, a guy with a huge engine to put out an enormous turn of speed for a couple km at the end. If Tony hadn’t crashed out of the Tour, however, he’d be one of the guys chasing breaks down, not in one himself, trying to help Cavendish win in Paris for an unprecedented 5th time.