My humble NextJS app is 100/100 on Lighthouse, I am really hesitant to believe that it scores so low
Scores need to be adjusted by and looked at through the lens of complexity. Next.js powers some of the most sophisticated web applications in the world, not just your blog and mine.
e.g.: the instant you bring "necessary evils" like display ads, your scores WILL suffer.
The bias we're seeing here is that NextJS is more popular and slow sites are being made with it, while people who choose Astro are more inclined to value the speed of their site.