But you can put him in a weaker car. That's exactly why Leclerc shouldn't get the Ferrari seat straight away. Vandoorne's lucky that this year's McLaren is exactly at the kind of level Leclerc should be right now, and even then he's doing his pre-F1 reputation no justice whatsoever.
Again, this is Ferrari. They don't need unknowns and they don't need pointless risks. They need a consistent #2 to Vettel, and Raikkonen can provide that just fine. With Leclerc they run the risk of him either slower, less consistent or less loyal, all of which would harm them, so promoting him straight away would be a very risky and most likely bad idea.
Except he, too, got 1.25 seasons in a slower car, and barring the fluke win, Ricciardo, clearly a less talented driver with less potential than Verstappen, trounced him the rest of the year. Problem is, Verstappen is not the norm, as Vandoorne has shown no promise whatsoever so far - he's still just as slow as he was at the start of the year, while Verstappen kept improving very noticeably from race to race.