A few different effects that culminate into a large perception. This is all opinion, so prepend every sentence here with "I think" or "in my opinion".

1. Most popular anime are high school themed. There certainly are anime with adults, but the majority is high school or younger.

2. Kids are cute. Young men growing into adulthood are in that weird metamorphosis phase where they used to be cute and could be silly without repercussions and now have to start fitting into the societal gender role that tells then they have to start taking things seriously. Online, no one knows you're a dog [meme drop]. They can hide behind a cute girl avatar and be silly.

3. So why not adults? See point 1. But also, humans tend to be in their prime around 18. I'm in my 30s, but wish I had some of the features I had when I was a teenager.

4. Dysphoria from a time you know: most people posting online are teenage or older. Their dysphoria is going to come from times they had the most free time: usually as a teenager before they had work and responsibilities. They don't know what it's like to be an adult yet, so they relate more to the teenage characters. School is ubiquitous, salary work less so.

In short, the actual answer likely lies somewhere in these few points, and will be a different mix for everyone. Some guys might have it for creepy reasons, but in the vast majority of cases it's because they feel a connection to the character, who happens to be a high school girl.