It's not about being ready to lead the line now. It's being part of the squad now while having the potential to be an elite CF. He's got big potential right? Well that's just the going rate for big potential with rare qualities.
Lewandowski's breakout season was in 2011/12 for Dortmund... he turned 23 at the start of that season. Dzeko's was 08/09 for Wolfsburg, he was 22 and turned 23 closer to the end of the season. Ibrahimovic was signed by Juve in 2004... shortly before he turned 23. Cavani's first half decent season at Palermo (15 goals) was in 08/09 and took til 10/11 to pass 20 goals, he turned 22 during that Palermo season. Kane broke out in 14/15, he turned 21 before that season. Giroud had his first big goalscoring season at age 23. Berbatov at 22. Mario Gomez 22. And so on.
Look at what all these guys did before that year when they made the big step up. Not even reaching 10 goals (apart from Ibra who had like 13 but it was for Ajax, so a bit different). It's not reasonable to expect a 20 year old to instantly step in and be a world class center forward, especially one that plays like that where they are big, invite physical duels with the defender but also focus on their technique. One thing if they are just a poacher or a smaller roaming forward to break through young, they can make an impact with their pace and agility, but much more difficult as a bigger guy. Haaland, yes is a freak of nature and an exception, but also he broke through as a pure poacher basically.
By comparison, Hojlund turned 20 in February. He scored just 1 goal for Atalanta between August and the new year. After the new year he scored 9 goals (showing a significant step up just around when he turned 20), and scored 6 goals for Denmark since as well... 15 goals for club and country in half a year having just turned 20 is pretty impressive and who knows how long it'd take him to adapt to the prem, but a year or 2 of being a squad player is fine (and I don't think it'd even take him that long given the recent jump).
Also to add to this if we want to look at more examples... how many bigger & technical strikers have there actually been in the last 20 years who were rated highly enough at a younger age for a club in a big league to go for him? Even the ones who have flopped. Someone like Weghorst was just never rated all that highly and only moved to Wolfsburg at 25, there just isn't all that many of those types of talents and the history from what I can seems to be some pretty damn good center forwards.