> it's probably react that causes all these spinning "loaders" everytime I click somewhere
Statistically speaking it's _probably_ jQuery or pure JavaScript. These loading spinners have been around to make site loading more attractive for a long time, predating React, Vue, Ember, even Backbone. I have tons of clients on Wordpress that love this kind of thing even if all it does is make their site load without a flash of unstyled content (FOUC).
> If the server would just send a rendered page
React is primarily a client-side framework, but it's possible to render it on the server using Node. However, a lot of sites still use React to enhance an existing frontend, and therefore do not port their entire server to Node to render with React.
> why would a loading animation be needed?
Alright so lets assume you've server-rendered with React, do you still need a loading animation? Maybe. All these sites you click on that have loading spinners are all server-rendered pages too, but that doesn't stop people from throwing jQuery plugins at it to make their sites look fancy and prevent a FOUC. It's more a question of how many images the site wants to display above the fold. If there's a bunch of images, your site will not load fast and you can choose between letting users see a gappy page gradually get filled in or sit and look at a spinner for 1.5s.
EDIT: not to mention all the advertising trackers these sites have enabled. This makes your image-loading all the more laggy.
You didn't ask a question, you jumped to a (wildly inaccurate) conclusion.