Kasparov was in a slump in 2004. He had recently lost to Deep Blue, had been WC for 15 years and wasn't hungry any more. However, after a 12+ year retirement, I think he's getting hungry again. We might see the old fighting spirit of Kasparov be re-kindled in St. Louis. I know he can beat Nakamura, because he's done it a few times. He's also beaten Caruana. Now that he's had some time to prepare, I'm hoping he can give So, Kramnik, Anand and Carlsen a run for their money.
BREAKING NEWS: Garry Kasparov comes out of retirement for Saint Louis GCT event!
He may have been in a slump, but still scored results like this one:
http://www.chessgames.com/perl/chess.pl?tid=43917

I don't know why they don't set up a Kasparov vs. Carlson showdown.
There are two ways a match between the two would happen: first, is if Garry were to unretire and win the Candidates tournament. Even Kaspy doesn't think that very likely.
The other would be some sort of rapid or blitz match sponsored by Sinquefield or chess.com. That is somewhat more likely, but only if Kaspy does well in this and a few other events.
Kasparov is intentionally avoiding most of the toughest competition in his first games back. Let's see how he does before declaring him the second best player in the World.

Who knows, didn't the second best player in the World win Linares tournament with a +3000 performance in 1994?

How many years ago was that?
23 years..... 12 years since his last competitive tournament. I expect major rust.
"Kasparov is intentionally avoiding most of the toughest competition"
Don't think so. He probably intended to play this event, and then had to settle with the weakie field of four blitz World Champions, Nakamura, Nepo, Anand etc...

I don't know why they don't set up a Kasparov vs. Carlson showdown.
There are two ways a match between the two would happen: first, is if Garry were to unretire and win the Candidates tournament. Even Kaspy doesn't think that very likely.
The other would be some sort of rapid or blitz match sponsored by Sinquefield or chess.com. That is somewhat more likely, but only if Kaspy does well in this and a few other events.
Kasparov is intentionally avoiding most of the toughest competition in his first games back. Let's see how he does before declaring him the second best player in the World.
You're assuming that Magnus is still the best player in the world. Kramnik, So, Aronian, Caruana and Mamedyarov are right on his @$$, as far as ratings go. I would put Kasparov in that group too. Yes, it appears that he is avoiding Carlsen, Kramnik and So, for now. Let's see if he still avoids them after some more training and a couple of tournaments under his belt.

How many years ago was that?
23 years..... 12 years since his last competitive tournament. I expect major rust.
I was meaning Karpov.

The highest rated person in classical chess is also the highest rated person in rapid and blitz chess and the reigning world champion in classical chess.
He's unquestionably the best player in the world. Right now, his competition is closing in on him, but they haven't caught him.
We don't know whether the peloton will reel in the leader or if the leader has an extra gear. I rather suspect Carlsen has extra gears.
"it appears that he is avoiding Carlsen, Kramnik and So"
He is playing one event where there are four blitz World Champions plus Nakamura, Anand and Nepo, all of them maybe even better than them at blitz... Then one can claim that he is avoiding the few top players that are not participating, but if they played instead of Nakamura or Anand or Karjakin people would probably be saying that he was intentionally avoiding Nakamura and Anand and Karjakin. And it's not as if So or Kramnik are better blitz players than for example Nakamura either.
So is #27 and Kramnik #16 on the blitz rating list, both well behind for example the less known Dominguez who is playing the event (and has won the World Blitz title).

I wonder if he came out of retirement after seeing Floyd Mayweather come out of it and thought to himself, "I wonder if I can come back and win.". lol

Kasperov cant beat magnus he could bearly beat top ten in blitz while magus dominate everyone and stomp them.. garry should never return he will lose his tititle as undisputed champion
Before the tournament, here are some great Kasparov achievements.
7-1 vs 4 2600 Israeli GMs in a simul.
50-0 vs Shirov, Adams, Gelfand, and Bareev.
Chessmatrics has 17 2820+ tournament performances for Kasparov.
Garry dominated for 20 years
Kasparov won many tournaments with all the best players present.
Back in 2004, when Kasparov still was very much active and clear #1, he played a blitz tournament in Reykjavik. There he faced a young Aronian, still just a promising player in the mid 2600s and far from the 2800+ player he has developed into. The result? Aronian won against Kasparov and won the tournament... So it's by no means easy for Kasparov to do much better today.
http://www.chessbanter.com/rec-games-chess-politics-chess/5213-levon-aronian-scores-big-upset.html