It takes a lot of work to create content. If you have a site with a lot of existing content on it, it’s probable that you have traffic coming to that content from different sources (other websites, search engines, cross-linking content from the same website). If you migrate domains, those links could break, so it’s important to add redirects, but if you have a LOT of content, it creates a LOT of work to do one-to-one redirect rules.