Frontend devs were largely bullied into imitating traditional MPAs by people posting memes about SPAs and loading spinners. A huge percentage of pages could simply be pre-rendered (e.g. static landing pages and documentation sites), or they're behind a login wall in which case SEO no longer really matters. Deploying an SPA used to mean uploading a few scripts and html documents to an S3 bucket, and with CDN caching it was fast enough for many organizations. Instagram is still a mostly 100% client side SPA.