It can, it’s just a lot of work. You’d be building device frames, hunting for backgrounds and fixing layouts for every shot. Moonjar gives it all of that, so you only have to ask.
Can’t this just be a skill?
Yes, but it’s really hard. You’d still have to find the backgrounds, frames and assets and tune it until it’s good, and a skill can’t remember what you like, keep every version or show you variations. Moonjar is built for screenshots and does all of it.
Is it free?
During the beta, yes. After that, most likely a free plan with some usage, and a Pro plan for $10–20 a month.