Polling companies assume (note that I didn't say "find in their results") that a very large portion of centrist voters will not back Melenchon in the second round. Pollsters almost never use raw data, and add weights, but the way they do this with Melenchon is not backed by the data: in the second round of parliamentary elections, when it was a two-way-race between a candidate from La France Insoumise (LFI, Melenchon's party), and a candidate of the Rassemblement National (RN, Le Pen's extreme-right party), a majority voted for LFI-candidates.
Furthermore, Melenchon's base is not with the people who vote in every electoin, but with people who have to be mobilised. You should check the polls for the first round of the previous 2 or 3 French presidential elections, and you'll see that he rises exponentially when he starts campaigning. The fact that he's already more or less surpassing the centrist candidates for second place in the first round, is very hopefull as this is happening much earlier than in previous elections.
Lastly, Melenchon is rethorically the strongest politician in the Global North, that is not hypeverbally. Nobody gives a speech like him, or debates like him. He will destroy anyone in a one-on-one debate if he makes it to the second round.