Why is Netlify not as popular as Vercel?Is it due to a lack of marketing?

Probably marketing but depends. Netlify is suited to static pages only and Vercel can do a lot more.

But to be honest, both of these don't make any sense as both of them are VERY expensive for what they offer, even when starting out because their service is mostly geared to VC backed startups and enterprises.

You can get very far by just having a VPS + Cloudflare that costs $1-10/mo even with 'one click apps' these days.

You don't need Netlify or Vercel.