It's for obviously insane reasons which is even more annoying.
1) It's outside of Europe in hot weather, a South American team will win it
2) Messi - it's in the stars
3) Their unbeaten run consisting of mainly friendlies, South American teams and a decimated Italy side
4) Copa America win - where Brazil trounced them, but they managed to fluke a goal.
Those are the excuses. The true reason is quite simple. We watch European teams and hardly watch Conmebol. Therefore, fans are far harsher critics of the European teams. Portugal, who people are still criticizing, just beat the toughest team outside of Brazil and Argentina in South America quite easily. That should tell you everything you need to know about that run. It's easy criticizing European teams for going out on penalties in the Euros to teams that are relatively strong.
People can criticize Holland's team with relatively strong midfielders and a Frenkie De Jong, but feel a team with Christian Romero and an aging Otamendi, Sevilla full backs, a Fred and Mctominay like midfield, McCallister ( who plays for brighton) playing on the left flank, an aging partially injured Di Maria and hardly any depth should be favored. Messi is the GOAT, but he's not in his prime either and shouldn't be expected to lift this squad when he couldn't do it with the likes of Mascherano, a younger Di Maria, Aguero, Tevez, Higuain etc
In saying all this, Argentina do have the potential to become a bit more formidable in some of their younger players, if they decide to use them. Lisandro Martinez, Alvarez, Enzo Fernandez too. The big issue is that midfield. Fix that and they actually have a chance.